Extensions of Mendelian Genetics

Extensions of Mendelian Genetics
Text authored by Dr. Peter J. Russell Slides authored by Dr. James R. Jabbur CHAPTER 12 Extensions of Mendelian Genetics

Chromosomes and Cellular Reproduction
Human somatic cells have 23 pairs of homologous chromosomes*** Homologous chromosomes are the same length and carry genes encoding similar inherited characteristics, with one chromosome contributed by each parent. Thus, the 46 total chromosomes in a human somatic cell are two sets of 23, with one set from the mother and one set from the father A diploid cell (2n) has two sets of chromosomes. For human somatic cells, the diploid number is 46, where 2 X 23 = 46. [n=23] The sex chromosomes (the 23rd chromosome) are called X and Y. Human females have a homologous pair of X chromosomes (XX). Human males have one X and one Y chromosome (XY) The other 22 pairs of chromosomes are called autosomes In a somatic cell in which DNA synthesis has occurred, each chromosome is replicated. Each replicated chromosome consists of two identical sister chromatids, producing a temporary tetraploid cell (4n) which undergoes mitosis to produce two diploid cells (2n) A karyotype is an ordered display of the pairs of chromosomes

replicated chromosomes
from dad Karyotype Analysis Chromosome pairs in diploid organisms are homologous. One member of each pair is inherited from each parent. Chromosomes that have different alleles are nonhomologous. from mom 5 µm Pair of homologous replicated chromosomes Tetraploid (4n) Centromere Figure 12.1 Chromosomal organization of haploid and diploid organisms. Sister Chromatids (from dad) Sister Chromatids (from mom) 23

The present slide shows Giemsa staining, with produced ‘G bands’
The prior slide displayed ‘chromosome painting’, involving in situ hybridization with fluorescently labeled DNA probes The present slide shows Giemsa staining, with produced ‘G bands’ Human chromosomes are numbered from largest to smallest and are also grouped (A-G) based on their banding pattern (morphology) Metaphase chromosomes show about 300 G bands, while prophase chromosomes show about 2,000 (called an ideogram) The smaller arm of the chromosome is designated p and the larger one as q (french?) Regions and subregions are numbered from the centromere outward (1 is closest) An example is the BRCA1 (breast cancer susceptibility) gene at 17q21 (long arm of chromosome 17 in region 21) Figure 12.3 G banding in a karyotype of human male metaphase chromosomes. a single sex chromosome.

Chromosomes differ in morphology based on the location of the centromere, which is employed in the attachment and segregation of chromosomes during mitosis and meiosis Figure 12.2 General classification of eukaryotic chromosomes as metacentric, submetacentric, acrocentric, and telocentric types, based on the position of the centromere.

Human gamete cells (sperm or egg) contain a single set of chromosomes, termed haploid (n)
For humans, the haploid number is 23 (n = 23) Each set of 23 consists of 22 autosomes and a single sex chromosome In an unfertilized egg (ovum), the sex chromosome is X In a sperm cell, the sex chromosome may be either X or Y Gametes are the only types of human cells produced by meiosis Meiosis results in one set of chromosomes in each gamete

Mitosis The process of eukaryotic cell division consists of…
Mitosis, the division of the nucleus, and Cytokinesis, the division of the cytoplasm The cell cycle consists of… Interphase, which is the copying of chromosomes and cell constituents in preparation for cell division Mitotic (M) phase, in which chromosomes partition (mitosis) and one cell divides into two (cytokinesis) Interphase (about 90% of the cell cycle) can be divided into three subphases of cell growth: G1 phase (“first gap”) – prepare for DNA synthesis S phase (“synthesis”) – duplicate DNA G2 phase (“second gap”) – prepare for M phase Figure 12.4 Eukaryotic cell cycle. This cycle assumes a period of 24 hours, although great variation exists between cell types and organisms.

Mitosis is further divided into five phases:
Prophase Prometaphase Metaphase Anaphase Telophase Although Cytokinesis is classified as occuring after Mitosis, it is well underway by late telophase BioFlix: Mitosis

Mitosis and Cytokinesis
G2 of Interphase Prophase Prometaphase Metaphase Anaphase Telophase and Cytokinesis Centrosomes (with centriole pairs) Chromatin (duplicated) Centrosome Centromere Fragments of nuclear envelope Nonkinetochore microtubules Metaphase plate Cleavage furrow Nucleolus forming Asters Nucleolus Nuclear envelope Plasma membrane Chromosome, consisting of two sister chromatids Kinetochore Kinetochore microtubule Mitotic Spindle Centrosome at one spindle pole Daughter chromosomes Nuclear envelope forming Duplicated cellular contents Chromosomes condense & Centrosomes begin to polarize Mitotic Spindle begins to form (asters develop) Nuclear envelope fragments apart & Centrosomes are polarized Chromosomes begin to align Mitotic Spindle is formed & Metaphase Plate Sister Chromosomes are pulled toward the poles at their centromeres Nuclear contents are separated and nucleus reforms; spindle disappears & Cell divides into 2 cells Protein scaffold causes condensation

Redux… Figure 12.5 Interphase and mitosis in an animal cell.

Microtubules (asters)
Force of Division Aster Centrosome Sister chromatids Microtubules (asters) Chromosomes Metaphase plate Kineto- chores Centrosome 1 µm Overlapping nonkinetochore microtubules Kinetochore microtubules Centrosome -> Microtubule -> Kinetochore -> Centromere -> Chromosomes 0.5 µm

Animation: Cytokinesis
Differences in Cytokinesis: A Closer Look Cleavage: animal cell Cell plate formation: plant cell (TEM) (SEM) Vesicles forming cell plate Wall of parent cell 1 µm 100 µm Cleavage furrow Cell plate New cell wall Figure 12.8 Cytokinesis (cell division). (a) Diagram of cytokinesis in an animal cell. (b) Diagram of cytokinesis in a plant cell. Contractile ring of microfilaments Daughter cells Daughter cells Animation: Cytokinesis

Summary for your review: Mitosis
The mitotic spindle is an apparatus of microtubules that controls chromosome movement during mitosis During prophase, assembly of spindle microtubules begins in the centrosome, the microtubule organizing center The centrosome replicates, forming two centrosomes that migrate to opposite ends of the cell, as spindle microtubules grow out from them An aster (a radial array of short microtubules) extends from each centrosome The spindle includes the centrosomes, the spindle microtubules, and the asters During prometaphase, some kinetochore microtubules attach to the kinetochores of chromosomes and begin to move the chromosomes At metaphase, the chromosomes are all lined up at the metaphase plate, the midway point between the spindle’s two poles In anaphase, sister chromatids separate and move along the kinetochore microtubules toward opposite ends of the cell The microtubules shorten by depolymerizing at their kinetochore ends Nonkinetochore microtubules from opposite poles overlap and push against each other, elongating the cell In telophase, genetically identical daughter nuclei form at opposite ends of the cell

Meiosis Like mitosis, meiosis is preceded by the duplication of chromosomes during interphase In contrast, meiosis takes place in two sets of cell divisions, called meiosis I and meiosis II, resulting in four different gamete cells rather than two identical daughter cells produced in mitosis (obviously, there is no DNA replication between meiosis I and II) Each final daughter cell has only half as many chromosomes (haploid, 1n) as the parent cell (diploid, 2n)

Tetraploid cell with replicated chromosomes
Interphase The Stages of Meiosis Homologous pair of chromosomes in diploid parent cell 2n diploid Diploid cell with unreplicated chromosomes Chromosomes replicate Homologous pair of replicated chromosomes 4n tetraploid Sister chromatids Tetraploid cell with replicated chromosomes Meiosis I 2n diploid Figure 13.7 Overview of meiosis: how meiosis reduces chromosome number 1 Homologous chromosomes separate Haploid cells with replicated chromosomes Meiosis II 2 Sister chromatids separate BioFlix: Meiosis 1n haploid Haploid cells with unreplicated chromosomes

The process of Meiosis Prophase I Metaphase I Anaphase I Telophase I and Cytokinesis Prophase II Metaphase II Anaphase II Telophase II and Cytokinesis Centrosome (with centriole pair) Sister chromatids remain attached Sister chromatids Centromere (with kinetochore) Chiasmata Spindle Metaphase plate Sister chromatids separate Haploid daughter cells forming Homologous chromosomes Homologous chromosomes separate Cleavage furrow Figure 13.8 The meiotic division of an animal cell Fragments of nuclear envelope Microtubule attached to kinetochore 1(4n) > 2(2n) > 4(1n)

Meiosis I crossover assortment I Prophase I Metaphase I Anaphase I
Telophase I and Cytokinesis Prophase I Metaphase I Anaphase I Centrosome (with centriole pair) Sister chromatids remain attached Centromere (with kinetochore) Sister chromatids Chiasmata Spindle Metaphase plate Figure 13.8 The meiotic division of an animal cell Cleavage furrow Homologous chromosomes Homologous chromosomes separate Fragments of nuclear envelope Microtubule attached to kinetochore crossover assortment I

Prophase I has distinct stages where homologous
chromosomes pair and undergo crossing-over Leptonema is when the chromosomes begin to coil, committing the cell to the meiotic process In Zygonema, the chromosomes are at their maximum condensation and synapse; a tight association between homologs occurs. Telomeres mediate the formation of a 4-chromatid synaptonemal tetrad During Pachynema, crossover (recombination) occurs Crossing-over is a reciprocal exchange of chromosome segments between homologues Diplonema is the period when chromosomes begin to move apart and the chiasmata (singular is chiasma) formed by crossing-over become visible Diakinesis involves the breakdown of the nucleoli and nuclear envelope, and assembly of the spindle. This is the phase where chromosomes are most easily counted Sex chromosomes are not homologous, but in some mammals they behave as if they were due to shared pseudoautosomal regions (PARs). The PAR regions enable crossing over between X and Y

Haploid daughter cells
Meiosis II Telophase II and Cytokinesis Prophase II Metaphase II Anaphase II Sister chromatids separate Figure 13.8 The meiotic division of an animal cell Haploid daughter cells forming assortment II

Summary for your review: Meiosis
Interphase preceeds Meiosis; chromosomes are replicated to form sister chromatids The sister chromatids are genetically identical and joined at the centromere The single centrosome replicates, forming two centrosomes Division in meiosis I occurs in four phases: – Prophase I – Metaphase I – Anaphase I – Telophase I and cytokinesis Prophase I typically occupies more than 90% of the time required for meiosis Chromosomes begin to condense In synapsis, homologous chromosomes loosely pair up, aligned gene by gene In crossing over, nonsister chromatids exchange DNA segments Each pair of chromosomes forms a tetrad, a group of four chromatids (bivalents) Each tetrad usually has one or more chiasmata, X-shaped regions where crossing over occurred For the Cell Biology Video Meiosis I in Sperm Formation, go to Animation and Video Files.

In metaphase I, tetrads line up at the metaphase plate (equatorial plane), with one chromosome facing each pole Microtubules from one pole are attached to the kinetochore of one chromosome of each tetrad Microtubules from the other pole are attached to the kinetochore of the other chromosome In anaphase I, pairs of homologous chromosomes (bivalents) separate One chromosome moves toward each pole, guided by the spindle apparatus Sister chromatids (dyads) remain attached at the centromere and move as one unit toward the pole In the beginning of telophase I, each half of the cell has a haploid set of chromosomes; each chromosome still consists of two sister chromatids Cytokinesis usually occurs simultaneously, forming two haploid daughter cells In animal cells, a cleavage furrow forms; in plant cells, a cell plate forms No chromosome replication occurs between the end of meiosis I and the beginning of meiosis II because the chromosomes are already replicated

Division in meiosis II also occurs in four phases:
– Prophase II – Metaphase II – Anaphase II – Telophase II and cytokinesis Meiosis II is very similar to mitosis In prophase II, a spindle apparatus begins to form In late prophase II, chromosomes (each still composed of two chromatids) move toward the metaphase plate In metaphase II, the sister chromatids are arranged at the metaphase plate Because of crossing over in meiosis I, the two sister chromatids of each chromosome are no longer genetically identical The kinetochores of sister chromatids attach to microtubules extending from opposite poles In anaphase II, the sister chromatids separate The sister chromatids of each chromosome now move as two newly individual chromosomes toward opposite poles

In telophase II, the chromosomes arrive at opposite poles
Nuclei form, and the chromosomes begin decondensing Cytokinesis separates the cytoplasm At the end of meiosis, there are four daughter cells, each with a haploid set of unreplicated chromosomes Each daughter cell is genetically distinct from the others and from the parent cell

Redux…

A Comparison of Mitosis and Meiosis
Mitosis conserves the number of chromosome sets, producing cells that are genetically identical to the parent cell Meiosis reduces the number of chromosomes sets from two (diploid) to one (haploid), producing cells that differ genetically from each other and from the parent cell The mechanism for separating sister chromatids is virtually identical in meiosis II and mitosis

Three events are unique to meiosis, and all three occur in meiosis l:
Synapsis and crossing over occurs during prophase I: Homologous chromosomes physically connect and exchange genetic information, facilitating genetic variation At the metaphase I plate, there are paired homologous chromosomes (tetrads), instead of individual replicated chromosomes At anaphase I, homologous chromosomes align randomly and separate, independently assorting, facilitating genetic variation

Figure 13.9 A comparison of mitosis and meiosis in diploid cells
Parent cell Chiasma MEIOSIS I Chromosome replication Chromosome replication Prophase Prophase I Homologous chromosome pair Replicated chromosome 2n = 6 Metaphase Metaphase I Anaphase Anaphase I Telophase Telophase I Haploid n = 3 Daughter cells of meiosis I 2n 2n MEIOSIS II Daughter cells of mitosis n n n n Daughter cells of meiosis II Figure 13.9 A comparison of mitosis and meiosis in diploid cells SUMMARY Property Mitosis Meiosis DNA replication Occurs during interphase before mitosis begins Occurs during interphase before meiosis I begins Number of divisions One, including prophase, metaphase, anahase, and telophase Two, each including prophase, metaphase, anaphase, and telophase Synapsis of homologous chromosomes Does not occur Occurs during prophase I along with crossing over between nonsister chromatids; resulting chiasmata hold pairs together due to sister chromatid cohesion Number of daughter cells and genetic composition Two, each diploid (2n) and genetically identical to the parent cell Four, each haploid (n), containing half as many chromosomes as the parent cell; genetically different from the parent cell and from each other Role in the animal body Enables multicellular adult to arise from zygote; produces cells for growth, repair, and, in some species, asexual reproduction Produces gametes; reduces number of chromosomes by half and introduces genetic variability amoung the gametes

Redux…

A Word on Cohesins… Protein complexes called cohesins are responsible for the cohesion of chromosomes (sister and homologous chromosomes) In mitosis, cohesins are cleaved at the end of anaphase, allowing sister chromosomes to segregate properly In meiosis, cohesins are cleaved along the chromosome arms in anaphase I (separation of homologous chromosomes) and at the centromeres in anaphase II (separation of sister chromosomes)

Origins of Genetic Variation Among Offspring
The behavior of chromosomes during meiosis and fertilization is responsible for most of the variation that arises in each generation Three mechanisms contribute to genetic variation: Independent assortment of chromosomes Crossing over Random fertilization Animation: Genetic Variation

Independent Assortment of Chromosomes
Homologous pairs of chromosomes orient randomly at metaphase I of meiosis In independent assortment, each pair of chromosomes sorts maternal and paternal homologues into daughter cells independently of the other pairs The number of combinations possible when chromosomes assort independently into gametes is 2n, where n is the haploid number For humans (n = 23), there are more than 8 million (223) possible combinations of chromosomes

Possibility 1 Possibility 2 Two equally probable arrangements of
chromosomes at metaphase I Metaphase II Figure The independent assortment of homologous chromosomes in meiosis Daughter cells Combination 1 Combination 2 Combination 3 Combination 4

Crossing Over Crossing over produces recombinant chromosomes, which combine genes inherited from each parent Crossing over begins very early in prophase I, as homologous chromosomes pair up gene by gene In crossing over, homologous portions of two nonsister chromatids trade places Crossing over contributes to genetic variation by combining DNA from two parents into a single chromosome

Recombinant chromosomes
Nonsister chromatids held together during synapsis Prophase I Pair of homologs Chiasma Centromere TEM Anaphase I Figure The results of crossing over during meiosis Anaphase II Daughter cells Recombinant chromosomes

Every human that ever lived (or will) is unique!
Random Fertilization Random fertilization adds to genetic variation because any sperm can fuse with any ovum (unfertilized egg) The fusion of two gametes (each with 8.4 million possible chromosome combinations from independent assortment) produces a zygote with any of about 70 trillion diploid combinations Every human that ever lived (or will) is unique!

Gamete production in animals
Spermatogeneisis in males: 4 haploid sperm are produced from the diploid spermatogonium Oogenesis in females: Each oogonium gives rise to one viable haploid ovum. Small polar bodies are formed, but they degenerate and are not viable In humans, this process is arrested at the end of meiosis I. When fertilization occurs, meiosis II is completed and the haploid sperm fertilizes the haploid ovum giving rise to the diploid zygote. Figure Spermatogenesis and oogenesis in an animal cell.

Gamete production in plants
Stamen: male reproductive structure, produces pollen Pistil: female reproductive structure, ovule produced Alternation of gametophyte and sporophyte generations Diploid multicellular sporophyte phase Haploid multicellular gametophyte phase .

Chromosomal theory of Inheritance
By the beginning of the 20th century, cytologists had observed that chromosome number is constant in all the cells of a species, but varies widely between species  Sutton and Boveri (1902) independently realized the parallel between Mendelian inheritance and chromosome transmission, and proposed the chromosome theory of inheritance, in which: Mendelian genes have specific loci on chromosomes Chromosomes undergo segregation and independent assortment

Sex Chromosomes The pioneering work of McClung, Stevens and Wilson indicated that chromosomes are different in male and female insects (X and Y chromosomes were denoted) In grasshoppers, all eggs have an X, and half of the sperm produced have an X, and the other half do not. After fertilization, an unpaired X produces a male, while paired X chromosomes produce a female In both humans and fruit flies (Drosophila melanogaster) females have two X chromosomes, while males have an X and a Y Males produce two kinds of gametes with respect to sex chromosomes (X or Y) and are called the heterogametic sex Females produce gametes with only one kind of sex chromosome (X) and are called the homogametic sex

Sex Linkage The first solid evidence associating a specific gene with a specific chromosome came from Thomas Hunt Morgan, an embryologist Morgan’s experiments with fruit flies provided convincing evidence that chromosomes are the location of Mendel’s heritable factors Morgan noted wild type (+), or normal phenotypes, in contrast to mutant, or alternative phenotypes, that were common in the fly population

Animation: X-Linked Inheritance
In one experiment, Morgan mated male flies with white eyes (mutant) with female flies with red eyes (wild type) The F1 generation all had red eyes, so white eye color is obviously recessive After breeding the F1 generation with each other, the F2 generation showed the 3 to 1, red to white eye ratio, but only males had white eyes Morgan determined that the white-eyed mutant allele must be located on the X chromosome, opening the discovery of sex- linkage and supporting the chromosome theory of inheritance Animation: X-Linked Inheritance

Figure The X-linked inheritance of red eyes and white eyes in Drosophila melanogaster. The symbols w and w indicate the white- and red-eyed alleles, respectively. (a) A redeyed female is crossed with a white-eyed male. (b) The flies are interbred to produce the F2s.

Morgan’s hypothesis was confirmed by an experiment reciprocal to the original cross
When a white-eyed female was crossed with a wild-type, red eyed male, the filial results differed from the original crosses All the F1 females had red eyes and all the males had white eyes These findings statistically supported the conclusion that the gene involved is likely to be X-linked (sex linked), providing strong evidence that genes are located on chromosomes

Figure 12. 19 Reciprocal cross of that shown in Figure 12. 18
Figure Reciprocal cross of that shown in Figure (a) A homozygous white-eyed female is crossed with a red-eyed (wild-type) male. (b) The F1 flies are interbred to produce the F2s. The results of this cross differ from those in Figure because of the way sex chromosomes segregate in crosses.

Nondisjunction of X Chromosomes
Calvin Bridges, a student of Morgan, found very rare (1/2000) exceptions in the mating between red eyed females and white eyed males Bridges’s hypothesis was that these exceptions are caused by the failure of chromatids to separate normally during anaphase of meiosis I or II, resulting in nondisjunction (primary or secondary) Animation: Nondisjunction

Primary and Secondary Nondisjunction
Figure Nondisjunction in meiosis involving the X chromosome. (Nondisjunction of autosomal chromosomes and of all chromosomes in mitosis occurs in the same way.) (a) Normal X chromosome segregation in meiosis. (b) Nondisjunction of X chromosomes in meiosis I. (c) Nondisjunction of X chromosomes in meiosis II.

(mono/tri -somic zygote) (mono/tri/tetra -ploid cell)
Primary nondisjunction Aneuploidy: one or more chromosomes of a normal set are missing or present in an unusual number (mono/tri -somic zygote) (mono/tri/tetra -ploid cell) Figure Rare primary nondisjunction during meiosis in a white-eyed female Drosophila melanogaster and results of a cross with a normal red-eyed male. Note the decreased viability of XXX and YO progeny.

Secondary Nondisjunction
Figure Results of a cross between the exceptional white-eyed XXY female of Figure with a normal red-eyed XY male. Again, XXX and YY progeny usually die. (a) Normal disjunction of the X chromosomes in the XXY female. (b) Secondary nondisjunction of the homologous X chromosomes in meiosis I of the XXY female.

Animation: Gene and Chromosome Segregation in Meiosis
Gene segregation mirrors chromosome behavior in meiosis. Mendel’s segregation and independent assortment of genes correlate with the movement of chromosomes during meiosis Animation: Gene and Chromosome Segregation in Meiosis

Sex Chromosomes and Sex Determination
Some mechanisms of sex determination include: Genotypic sex determination, in which sex is governed by genotype Genic sex determination, in which sex chromosomes are not involved

Genotypic Sex Determination
Sex Determination in Mammals In humans and many other animals, there is a chromosomal basis of sex determination, with a larger X chromosome and a smaller Y chromosome Only the ends of the Y chromosome have regions that are homologous with the X chromosome, allowing them to interact with each other in a homologous manner The SRY gene on the Y chromosome codes for the development of testes (I am quite fond of my SRY gene) Evidence for the Y chromosome mechanism of sex determination came from aneuploidies (next slides)

Turner’s: XO

Klinefelter’s: XXY

Porn Star

Dosage Compensation Mechanism for X-Linked Genes
Gene dosage varies between the sexes in mammals, because females have two copies of the X chromosome while males have only one copy Thus, early in development, gene expression from the X chromosome must be equalized to avoid death In females, one of the two X chromosomes becomes highly condensed, causing transcriptional deactivation This leaves one X chromosome that is equivalent to the single X chromosome of the male The deactivated X chromosome is randomly chosen in each cell for condensation early in development (in humans, Barr bodies form on day 16, postfertilization) Descendants of that cell will have the same X inactivated, making female mammals genetic mosaics (super cool!) i.e. calico coated cats - mosaic coat pattern

Figure Barr bodies. (a) Nuclei of normal human female cells (XX), showing Barr bodies (indicated by arrows). (b) Nuclei of normal human male cells (XY), showing no Barr bodies.

X chromosomes Allele for orange fur Early embryo: Allele for black fur Cell division and X chromosome inactivation Two cell populations in adult cat: Active X Inactive X Active X Black fur Orange fur Epigenetic Phenomena (heritable change in gene expression not caused by a change in the DNA sequence)

Sex Determination in Drosophila and Caenorhabditis
An X chromosome–autosome balance system is used, where sex is determined by the ratio between the number of X chromosomes and the number of sets of autosomes Drosophila has three pairs of autosomes and one pair of sex chromosomes Like humans, XX is female; XY is male. Unlike humans, Y does not determine sex, but it is required for male fertility In brief: The sex of the fly results from the ratio of the number of X chromosomes (X) to the number of sets of autosomes (A) – heck if I know what that ratio is!

Genotypic Sex Determination in Other Organisms is variable and
44 + XY 44 + XX Parents 22 + X 22 + Y 22 + X Genotypic Sex Determination in Other Organisms is variable and sometimes opposite that of mammals, with the male homogametic and the female heterogametic or + Sperm Egg 44 + XX 44 + XY or Zygotes (offspring) (a) The X-Y system 22 + XX 22 + X (b) The X-0 system 76 + ZW 76 + ZZ (c) The Z-W system 32 (Diploid) 16 (Haploid) (d) The haplo-diploid system

Genic sex determination
Other eukaryotes use a genic system instead of entire sex chromosomes. A single allele determines the mating type An example is the MATa and MATa allele in Saccharomyces cerevisiae Yeast mating types have identical morphologies, but are able to fertilize gametes only from the opposite mating type

Eukaryotic example: Mating in yeast  factor Receptor Exchange
1 Exchange of mating factors a  a factor Yeast cell, mating type a Yeast cell, mating type  Mating a  2 New a/ cell a/ 3

Analysis of Sex-Linked Traits in Humans
Sex-linked traits can be analyzed like autosomal ones; you use a pedigree There are inherent difficulties in pedigree analysis (data collection, sample size, trait expression…)

X-linked recessive inheritance
Human traits involving recessive alleles on the X chromosome are X-linked recessive traits X-linked recessive traits occur more frequently in males, due to hemizygousity. A female will only express an X-linked recessive trait if she were homozygous recessive at that locus Characteristics of X-linked recessive inheritance include: Affected fathers transmit the recessive allele to all of their daughters (who are therefore carriers) and to none of their sons Many more males than females exhibit the trait (hemizygousity) All sons of affected (homozygous recessive) mothers are expected to show the trait With a carrier mother, ½ of her sons will show the trait and ½ will be free of the allele A carrier female crossed with a normal male will have ½ of her daughters as carriers and the other ½ normal

Figure 12. 28 X-linked recessive inheritance
Figure X-linked recessive inheritance. (a) Painting of Queen Victoria as a young woman. (b) Pedigree of Queen Victoria (III-2) and her descendants, showing the inheritance of hemophilia. (See Figure 11.16, p. 289, for an explanation of symbols used in pedigrees. In the pedigree shown here, marriage partners who were normal with respect to the trait may have been omitted to save space.) Since Queen Victoria was heterozygous for the sex-linked recessive hemophilia allele, but no cases occurred in her ancestors, the trait may have arisen as a mutation in one of her parents’ germ cells (the cells that give rise to the gametes).

X-linked dominant inheritance
Patterns of inheritance are the same as X-linked recessives, except that heterozygous females will show the trait Only a few X-linked dominants are known. Examples include: Hereditary enamel hypoplasia (faulty and discolored tooth enamel) Webbing to the tips of the toes Constitutional thrombopathy (severe bleeding due to lack of blood platelets)

Y-Linked Inheritance With exception to maleness (SRY), Y-linked (holandric) traits have not been confirmed (but many genes have been identified on the Y chromosome The hairy ears trait may be Y linked, but it is a complex phenotype, and might also be the result of autosomal gene(s) and/or effects of testosterone

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