3 Sex-Linked Inheritance Basic idea: any gene located on the X chromosome (in mammals, Drosophila and others) or on the analogous Z chromosome (in birds and other species) is said to be sex-linked or X-linked.First sex-linked gene found was in Drosophila and it was the recessive white-eye mutation.A cross between white-eyed females (Xw Xw) with wild-type (red-eyed) males (X+ Y) produced all male offspring that were white-eyed like their mother and all the female offspring had red eyes like their father.This criss-cross mode of inheritance is characteristic of sex-linked genes.
4 Sex-Linked Inheritance (cont’d) Why is this crisscross mode characteristic?The Y chromosome carries no alleles homologous to those at the white locus on the X chromosomeX chromosome carries some genes, in most organisms, the Y contains only several dozen. So the males carry only one allele for sex-linked traits.This one allelic condition is called “hemizygous.”
5 Sex-Linked Inheritance (cont’d) Example: Determine the offspring in a cross between a heterozygous, red eyed female and a white eyed male.X+ Xw x Xw YX+XwX+ XwRed femaleXwXwWhite femaleYX+ YRed maleXw YWhite male
6 Sex-Linked Inheritance (cont’d) The reciprocal cross, where the sex-linked mutation appears in the male parent, results in the disappearance of the trait in the F1 and its reappearance only in the the males of the F2. This type of skip also indicates sex-linked genes.Example: Determine the F1and F2 offspring from a P1 cross between a:X+ X+ x Xw Yred female white male
7 X+ Xw X+ X+ Red female X+Xw red female Y X+ Y Xw Y Gametes: X+ Xw Y F1: X+ Xw red females and X+ Y malesso the white eyed males have disappeared.F2:X+XwX+ X+Red femaleX+Xwred femaleYX+ YRed maleXw YWhite maleOnly the males are showing the mutant trait (white eyes)
8 Sex-Linkage SummaryIf sex-linked recessive:it is usually found more frequently in the male than in the female of the speciesit fails to appear in females unless it also appeared in the paternal parent.it seldom appears in both father and son, then only if the maternal parent is heterozygous.
9 Sex-Linkage SummaryIf sex-linked dominant:it is found more frequently in the female than in the maleit is found in all female offspring of a male that shows the traitit fails to be transmitted to any son from a mother that did not exhibit the trait herself.
10 Figure 15.1 The chromosomal basis of Mendel’s laws
13 Non-Sex linked genesLinked genes refers to genes that are on the same chromosome and tend to be inherited together.Linked genes do not follow Mendel’s Law of Independent AssortmentLinked genes do not assort independently because they are located on the same chromosome and tend to move together through meiosis.Since the genes are linked, we would expect them to be passed on to the offspring together and the offspring would not have any different combinations than the parents.But, recombination can occur between linked genes.
14 Figure 15.4 Evidence for linked genes in Drosophila Since the observed ratio of the phenotypes was not 1 : 1 : 1 : 1, Morgan suspected that some genes are transmitted together because they are on the same chromosome.The offspring that were like the parents demonstrated linked genes but the recombined phenotypes suggested something else- crossing over- was occurring
15 Figure 15.5a Recombination due to crossing over
16 Figure 15.5b Recombination due to crossing over
17 Recombination Frequencies Where Are Those Gene’s Located? A Genetic MapIt was hypothesized that the recombination frequencies reflected the distances between genes on a chromosome.It was predicted that the farther away two genes were from each other, the higher the probability they would crossover and therefore the higher the recomb. frequency.Since the genes were farther away, there are more places or points between them where a cross over can occur.So a linkage map can be constructed from knowing how often genes cross over.
18 Figure 15.6 Using recombination frequencies to construct a genetic map An Example: 3 genes: body color (b), wing size (vg), eye color (cinnabar, cn, which is bright red). The recombination frequency between b and cn is 9% and between cn and vg, 9.5%.Multiple crossovers cause the frequency between b and vg to not equal the sum of %.One map unit = 1% cross over frequency
19 Figure 15.7 A partial genetic map of a Drosophila chromosome
20 Figure 15.8 Some chromosomal systems of sex determination X-O System: grasshoppers have only one type of sex chromosome. Males have only one sex chromosome and thus are XO.Z-W System: birds; the sex determiner is in the egg, not the sperm and thus the chromosomes are labeled Z and W; Males are ZZ and females are ZWBees and ants have no sex chromosomes; female bees develop from fertilized eggs (2n) and the males are from unfertilized eggs and are haploid
21 Figure 15.9 The transmission of sex-linked recessive traits
22 Determination of Sex Anatomical indications at about 8 weeks Sex-determining region YSRY gene on the Y chromosome is responsible for development of the testes.SRY codes for a protein called the testis-determining factor or TDF. This protein controls the expression of many other genes involved in testicular development and sperm production.
23 Fathers pass sex-linked alleles to all daughters but none of sons. Mothers can pass sex-linked alleles to sons and daughters.If the sex-linked trait is recessive, the female must be homozygous but the males will only need one copy and are called hemizygous. Therefore, more males express the trait/disorder than females.
24 X-linked Genes There are about 1000 X-linked genes. Most code for something other than female anatomical traits.Some of the non-sex determining X-linked genes are responsible for hemophilia, red-green color blindness, congenital night blindness, high blood pressure, duchene muscular dystrophy and fragile X syndrome.
25 Y-linked GenesThe Y chromosome is not only much smaller but also has about 26 genes and gene families.Most of these genes are involved with general cellular activities.9 genes are involved with sperm production.When all 9 genes are missing or defective, the result is very low sperm counts and infertility.It is not thought that about 1/3 of infertile couples are unable to have children as a consequence of the male mate not having the necessary sperm producing genes on his Y chromosome.SRY gene is responsible for male anatomical traits.
26 Sex-linked Disorders in Humans Duchenne Muscular Dystrophyweakening of muscles and loss of coordination due to loss of a muscle protein called dystrophin, coded by a gene on X chromosome.people live into their early 20s.Hemophiliasex-linked recessivemodern treatment is to inject those afflicted with the missing clotting protein.females tend to be carriers
27 Sex-linked Disorders in Humans cont’d X Inactivation in Female MammalsOne X chromosome in every cell is inactive so a female has one copy active, like a male but it is not the same X chromosome in all of her cells.Barr bodies: condensed, inactive X chromosome attached to nuclear membrane.Occurs randomly in embryonic cells so females are a mosaic of two types of cells. All mitotic divisions after the inactivation have the same inactive X chromosome.Calico or tortoiseshell catsMethyl groups on cytosine seem to cause the inactivation of one of the two X chromosomes. Which chromosome is chosen is determined by a gene making RNA that binds to the X chromosome that will then be inactive.
28 Figure 15.10 X inactivation and the tortoiseshell cat
30 Goof-Ups in Chromosomal Structure and Number Aneuploidy: abnormal chromosomal number due to nondisjunction.Trisomy: having 3 of that chromosome in a cellMonosomic: having one less than the expected chromosomal number.Polyploidy: an extra set of chromosomesTriploidy (3n) or tetraploidy (4n)Triploidy can occur if a diploid egg cell is fertilized so that means that all of the chromosomes went through nondisjunction.Tetraploidy: a 2n zygote duplicates all its chromosomes and then fails to divide giving you a 4n cell that then undergoes mitosis.
31 Goof-Ups in Chromosomal Structure and Number cont’d Alterations in structurea) Deletion: a piece of a chromosome is lost and therefore the cell containing that chromosome and all its descendants will be missing certain genes.b) Duplication: if this piece that is lost attaches to another (sister chromatid) then you have a duplication because the receiving sister chromatid has the chromosomal segment and then it also gets the lost piece.c) Inversion: a lost chromosomal fragment reattaches in the reverse order.d) Translocation: a lost piece attaches to a nonhomologous chromosome.
32 Figure 15.13 Alterations of chromosome structure
35 Human DisordersDown’s Syndrome1 out of every 700 children borntrisomic 21altered facial features, short height, heart defects, mental retardation.a concern for pregnant mothers over age of 30; fetal testing for trisomy 21 in the embryo
37 Human DisordersKlinefelter’s SyndromeXXY maleone out of every 2000 birthssmall testes and sterilemay include breast enlargementSuperfemales or metafemalesXXX with 2 Barr bodiescan be normal fertile women to physiological defects
39 Human DisordersTurner’s SyndromeXO femaleshort stature, webbing of neck skin, underdeveloped gonadsXYY Malesalso misnamed as “tall-aggressive syndrome”More XYY males have been found among the non- institutionalized population.subnormal IQs which may contribute to impulsive behavior.
41 Genomic ImprintingDefinition: a gene is silenced and will have different effects depending on whether it is in a male or a female regardless of whether the allele was delivered in a sperm or an egg.Methyl groups are added to cytosine nucleotides which prevents the transcription of a gene. The organism uses the product from the unaffected allele.about 20 mammalian genes subject to imprintingFragile X Syndrome: the tips of an X chromosome appear to hang onto the rest of the chromosome.1 out of every 1500 males and one of every 2500 femalesmental retardation is the result
45 Figure 15.16 Cytoplasmic inheritance in tomato leaves
46 Non-mendelian Inheritance (cytoplasmic inheritance) Not all eukaryotic genes are on nuclear DNASome genes are in the mitochondria and chloroplastsThese genes are transmitted to the “daughter” organelles.Mendelian inheritance is not demonstrated here.Example: in plants, genes controlling coloration in the plastids all come from the egg and not from the pollen. The coloration in the leaves is therefore due to the maternal plastid genes only.Example: mitochondrial genes in humans (and other mammals) are all maternal. Mitochondria in the sperm do not enter the egg at the time of fertilization.
47 Mitochondrial mutations show up in the proteins making up the electron Mitochondrial mutations show up in the proteins making up the electron transport chain and ATP synthase.Nervous and muscular systems are the most susceptible to energy losses so mitochondrial diseases show up the most in these body systems.
48 Meiosis reduces chromosome number and rearranges genetic information. Explain how the reduction and rearrangement are accomplished in meiosis.Several human disorders occur as a result of defects in the meiotic process. Identify ONE such chromosomal abnormality; what effects does it have on the phenotype of people with this disorder? Describe how this abnormality could result from a defect in meiosis.
49 REDUCTIONhomologous chromosomes pair, then separate anda move to opposite poles during first meiotic division.sister chromatids or chromatids separate during the second meiotic division.REARRANGEMENTcrossing over occursindependent assortment of the tetrads (random alignment)
50 Phenotype description CHROMOSOMAL ABNORMALITYIdentify ONE chromosomal abnormalityDown syndrome, XO or Turner; Fragile XPhenotype descriptionDESCRIBE- name or identify the meiotic eventNondisjunction for Down’s SyndromeNondisjunction for Turner (sex chromosome)Cri du chat ( deletion in chromosome 5)
51 DESCRIBE- description of the meiotic event Nondisjunction for Down’s SyndromeNondisjunction for Turner (sex chromosome)Cri du chat ( deletion in chromosome 5)