Chapter 10 Patterns of Inheritance.

Chapter 10 Patterns of Inheritance

What Is the Physical Basis of Inheritance?
Inheritance is the process by which traits are passed to the offspring Genes are sequences of nucleotides at specific locations on chromosomes A gene is a unit of heredity that encodes information needed to produce proteins, cells, and entire organisms The location of a gene on a chromosome is called its locus (plural, loci) Focus on diploid organisms that utilize sexual reproduction by fusion of gametes. Chromosome – single double helix of DNA

DNA in Eukaryotic Chromosomes
Homologous chromosomes are usually not identical Mutations - Changes in nucleotide sequence in DNA of one homologous chromosome Mutations are the raw material for evolution Mutations in gametes may become a part of the genetic makeup of the species Mutation gives rise to new alleles, alternative gene forms that may produce differences in structure or function Chromosomes occur in pairs A cell might make a mistake when it copies the DNA of one homologue but not the other. Or a carcinogen from the environment. make one homologue different genetically than the other 2. May be beneficial, harmful, or neutral to the organism 4. creating alleles – different hair colors, songs in birds For a given gene on two homologous chromosomes, both homologues may carry the same allele for the gene or may carry different alleles gene 1 gene 2 same alleles different alleles

The Relationship Among Genes, Alleles, and Chromosomes
Both chromosomes carry the same allele of the gene at this locus; the organism is homozygous at this locus This locus contains another gene for which the organism is homozygous Each chromosome carries a different allele of this gene, so the organism is heterozygous at this locus a pair of homologous chromosomes gene loci the chromosome from the male parent from the female An organism’s two alleles may be the same or different Each cell carries two alleles per characteristic, one on each of the two homologous chromosomes If both homologous chromosomes carry the same allele (gene form) at a given gene locus, the organism is homozygous at that locus If two homologous chromosomes carry different alleles at a given locus, the organism is heterozygous at that locus (a hybrid) Fig. 10-1

How Were the Principles of Inheritance Discovered?
Who was Gregor Mendel? Pea plants have qualities that make it a good organism for studying inheritance Pea flower petals enclose both male and female flower parts and prevent entry of pollen from another pea plant – self-fertilization intact pea flower flower dissected to show its reproductive structures Carpel (female, produces eggs) Stamen (male, produces pollen that contain sperm) Mendel was an Austrian monk in a monastery in the late 1800s He discovered the common patterns of inheritance and many essential facts about genes, alleles, and the distribution of alleles in gametes and zygotes during sexual reproduction Pea flowers have stamens, the male structures that produce pollen, that in turn contain the sperm (male gametes); sperm are gametes and pollen is the vehicle Pea flowers have carpels, female structures housing the ovaries, which produce the eggs (female gametes)

How Are Single Traits Inherited?
The language of a genetic cross A genetic cross is the mating of pollen and eggs (from same or different parents) The parents used in a cross are part of the parental generation (known as P) The offspring of the P generation are members of the first filial generation (F1) Offspring of the F1 generation are members of the F2 generation To see what characteristics a pea plant would inherit, Mendel used genetic crosses.

Cross of Pea Plants True-Breeding for White or Purple Flowers
pollen Parental generation (P) pollen cross-fertilize true-breeding, purple-flowered plant true-breeding, white-flowered plant First-generation offspring (F1) all purple-flowered plants Fig. 10-4

Self-Fertilization of F1 Pea Plants with Purple Flowers
First- generation offspring (F1) self-fertilize Second- generation offspring (F2) The capacity to produce white flowers had not disappeared in the F1 plants, but was only hidden. The F2 generation was allowed to self-fertilize. F3 generation – white flowers produced white flowers every time. But purple flowers produced purple or white. 3/4 purple 1/4 white Fig. 10-5

The Distribution of Alleles in Gametes
homozygous parent (a) Gametes produced by a homozygous parent (b) Gametes produced by a heterozygous dominant parent A gametes heterozygous parent a Each trait is determined by pairs of genes (two alleles). Dominant alleles mask the expression of recessive alleles. Alleles separate during meiosis. These are Mendel’s hypothesis. Fig. 10-6

Segregation of Alleles and Fusion of Gametes
purple parent PP P + P all P sperm and eggs white parent pp p + p all p sperm and eggs (a) Gametes produced by homozygous parents Fig. 10-7a

F1 offspring sperm eggs P + p Pp or p + P Pp (b) Fusion of gametes produces F1 offspring Fig. 10-7b

pp p + Pp gametes from F1 Pp plants eggs F2 offspring P PP sperm (c) Fusion of gametes from the F1 generation produces Fig. 10-7c

How Are Single Traits Inherited?
The hypothesis explains Mendel’s results with peas The particular combination of the two alleles carried by an individual is called the genotype The physical expression of the genotype is known as the phenotype (for example, purple or white flowers) Simple “genetic bookkeeping” can predict genotypes and phenotypes of offspring The Punnett square method predicts offspring genotypes and phenotypes from combinations of parental gametes We accept Mendel’s hypothesis as true based on the results.

Determining the Outcome of a Single-Trait Cross
Pp self-fertilize 1 P eggs 1 p 2 2 offspring genotypes genotypic ratio (1:2:1) phenotypic ratio (3:1) sperm eggs 1 P 2 1 P  1 P  1 2 4 PP 1 4 PP 2 Genotypic Ratio = 1:2:1 Phenotypic ratio = 3:1 We can use this method to figure out one of the alleles of a parent to find out if the parent is heterozygous or homozygous for a trait. 1 1 sperm 4 PP 4 Pp 1 P  1 p  1 2 Pp 2 4 3 4 purple 1 2 Pp 1 1 p p  1 P  1 pP 2 2 4 2 1 p  1 p  1 pp 1 1 1 pp 1 white pP pp 2 2 4 4 4 4 4 (a) Punnett square of a single-trait cross Using probabilities to determine the offspring of a single-trait cross Fig. 10-8

Traits of Pea Plants Studied by Gregor Mendel
Dominant form Recessive form Seed shape smooth wrinkled Seed color yellow green Pod shape inflated constricted Pod color green yellow Flower color purple white Flower location at leaf junctions at tips of branches Plant size tall (about 6 feet) dwarf (about 8 to 16 inches) Fig

Predicting Genotypes and Phenotypes for a Cross between Parents That Are Heterozygous for Two Traits
1 4 3 16 SY SSYY SsYY ssYY ssyY SsyY SSYy SsYy self-fertilize ssYy ssyy Ssyy SSyy sSyY sSyy sSYY sSYy SSyY sY sy Sy eggs seed shape seed color phenotypic ratio (9:3:3:1) smooth yellow 9 smooth yellow smooth green wrinkled yellow wrinkled green green wrinkled   (a) Punnett square of a two-trait cross (b) Using probabilities to determine the offspring of a two-trait cross sperm Fig Ratio = 9:3:3:1

How Are Multiple Traits Inherited?
Mendel’s results supported his hypothesis that traits are inherited independently The independent inheritance of two or more traits is called the law of independent assortment Multiple traits are inherited independently because the alleles of one gene are distributed to gametes independently of the alleles for other genes Independent assortment will occur when the traits being studied are controlled by genes on different pairs of homologous chromosomes

Independent Assortment of Alleles
pairs of alleles on homologous chromosomes in diploid cells Y y chromosomes replicate replicated homologous pair during metaphase of meiosis I, orienting like this or like this S Y S y s y s Y meiosis I S Y s y S y s Y During Meiosis I, lining up and separating is completely random. S Y s y S y s Y meiosis II S S s s S S s s Y Y y y y y Y Y SY sy Sy sY Fig independent assortment produces four equally likely allele combinations during meiosis

Do the Mendelian Rules of Inheritance Apply to All Traits?
Many traits do not follow simple Mendelian rules of inheritance Not all traits are completely controlled by a single gene Not all traits have only two possible alleles A trait may not be completely dominant to another

eggs father C1C2 C2C2 C1C1 mother sperm Incomplete dominance When the heterozygous phenotype is intermediate between the two homozygous phenotypes Hair texture In the genes studied by Mendel, one allele was dominant over the other, which was recessive Some alleles, however, are incompletely dominant over others

A single gene may have multiple alleles A species may have multiple alleles for a given characteristic However, each individual still carries two alleles for this characteristic The human blood types are an example (Codominance) Blood Type (Phenotype) Genotype (alleles) Antibodies in Plasma A AA or AO Anti-B B BB or BO Anti-A AB None O OO Anti-A and Anti-B

Polygenic inheritance Some characteristics show a range of continuous phenotypes instead of discrete, defined phenotypes Examples of this include human height, skin color, and body build, and in wheat, grain color Phenotypes produced by polygenic inheritance are governed by the interaction of more than two genes at multiple loci Height is controlled by 180 genes.

Polygenic Inheritance of Skin Color in Humans
Fig Human skin color is controlled by at least three genes, each with pairs of incompletely dominant alleles

The environment influences the expression of genes Newborn Siamese cats demonstrate the effect of environment on phenotype A Siamese cat has the genotype for dark fur all over its body However, the enzyme that produces the dark pigment is inactive at temperatures above 93°F (34°C) The environment in which an organism lives profoundly affects its phenotype Skin color and height In the womb, the baby is warm all over  inactive enzyme  born all white After birth, certain areas of the body remain cooler  activates the enzyme  darker color

How Are Genes Located on the Same Chromosome Inherited?
Genes on the same chromosome tend to be inherited together Mendel’s law of independent assortment works only for genes whose loci are on different pairs of homologous chromosomes Characteristics whose genes tend to assort together are said to be linked Example from a sweet pea (not Mendel’s edible pea) Does NOT conform to Mendel’s law of independent assortment flower-color gene pollen-shape gene purple allele, P long allele, L red allele, p round allele, l

How Are Genes Located on the Same Chromosome Inherited?
Crossing over creates new combinations of linked alleles Genes on the same chromosome do not always sort together Crossing over, or genetic recombination, in prophase I of meiosis creates new gene combinations flower-color gene purple allele, P long allele, L red allele, p round allele, l pollen-shape gene sister chromatids homologous chromosomes (duplicated) at meiosis I (a) Replicated chromosomes in prophase of meiosis I (b) Crossing over during prophase I P p L l One or more regions where crossing over occurs.

Crossing Over Recombines Alleles on Homologous Chromosomes
recombined chromatids P L p l unchanged (c) Homologous chromosomes separate at anaphase I chromosomes (d) after meiosis II The farther apart the two genes, the more likely crossing over will separate them. Thus causing them to seem independent assorted. Each being a different gamete with different alleles Fig c, d

How Is Sex Determined? Sex chromosomes that dictate gender
Y chromosome How Is Sex Determined? Sex chromosomes that dictate gender Females have two X chromosomes Males have an X chromosome and a Y chromosome A small section of the X and Y chromosomes is homologous, allowing them to pair in prophase I and segregate during meiosis Autosomes - rest of the (non-sex) chromosomes occur in identical pairs

Sex Determination in Mammals
eggs female parent female offspring male offspring male parent X1 X2 Xm Y sperm Gender of the offspring is determined by the sperm (father). Fig

How Are Sex-Linked Genes Inherited?
Sex-linked genes are found only on the X or only on the Y chromosome Genes carried on one sex chromosome, but not on the other, are sex-linked In humans, the X chromosome is much larger than the Y and carries over 1,000 genes In contrast, the human Y chromosome is smaller and carries only 78 genes During embryonic life, the action of the Y-linked gene SRY sets in motion the entire male developmental pathway Under normal conditions, SRY causes the male gender to be linked 100 percent to the Y chromosome

Sex-linked genes are found only on the X or only on the Y chromosomes Few of the genes on the X chromosome have a specific role in female reproduction Most of the genes on the X chromosome have no counterpart on the Y chromosome Some genes found only on the X chromosome are important to both sexes, such as genes for color vision, blood clotting, and certain structural proteins in muscles

Sex-linked genes are found only on the X or only on the Y chromosomes Because females have two X chromosomes, recessive sex-linked genes on an X chromosome may or may not be expressed Because males, with only one X chromosome, have no second copy to mask recessive genes, they fully express all the X-linked alleles they have, whether those alleles are dominant or recessive

Sex-Linked Inheritance of Color Blindness
eggs female parent female offspring male offspring male parent XC Xc Y (a) Normal color vision (b) Red-green color blindness (c) Expected children of a man with normal color vision (CY), and a heterozygous woman (Cc) Can’t distinguish red from green sperm Color deficiency One allele may be completely missing or a defective allele may cause both sets of cones to be sensitive to the same color. A person would not be able to distinguish red from green. Fig

Family Pedigrees - show the genetic relationships among relatives
How are Human Genetic Disorders Inherited? Family Pedigrees - show the genetic relationships among relatives (a) A pedigree for a dominant trait (b) A pedigree for a recessive trait How to read pedigrees = generations = male = female = parents = offspring or = shows trait = does not show trait = known carrier (heterozygote) for recessive trait = cannot determine the genotype from this pedigree ? We can’t do experimental genetic crosses on humans. So geneticists study family records. Fig

How Are Human Disorders Caused by Single Genes Inherited?
Some human genetic disorders are caused by recessive alleles New alleles produced by mutation usually code for nonfunctional proteins Alleles coding for nonfunctional proteins are recessive to those coding for functional ones The presence of one normal allele may generate enough functional protein to enable heterozygotes to be phenotypically indistinguishable from homozygotes with two normal alleles

Some human genetic disorders are caused by recessive alleles Heterozygous individuals are carriers of a recessive genetic trait (but otherwise have a normal phenotype) Recessive genes are more likely to occur in a homozygous combination (expressing the defective phenotype) when related individuals have children Sickle-cell anemia Mutation in hemoglobin structure Malaria resistance in heterozygotes It is not as likely for a non-related person to carry the same recessive alleles.

Albinism Fig. 10-22 Results from a defect in melanin production.
Recessive defect Fig

Some human genetic disorders are caused by dominant alleles A dominant disease can be transmitted to offspring if at least one parent suffers from the disease and lives long enough to reproduce Dominant disease alleles also arise as new mutations in the DNA of eggs or sperm of unaffected parents Dominant allele codes for an abnormal protein that interferes with the functional protein. or the new protein may carry out new, toxic reactions. or code for a protein that is overactive  performing its function at inappropriate times and places in the body.

Some human genetic disorders are caused by dominant alleles Huntington disease is a dominant disorder that causes a slow, progressive deterioration of parts of the brain The disease results in a loss of coordination, flailing movements, personality disturbances, and eventual death The disease becomes manifest in adulthood, ensuring its maintenance in the population The symptoms don’t show up until yrs of age.

Some human genetic disorders are sex-linked The X chromosome contains many genes that have no counterpart on the Y chromosome Because males have only one X chromosome, they have no other allele to exert dominance over a sex-linked (X-linked) allele causing disease Consequently, sex-linked diseases tend to occur in males

Some human genetic disorders are sex-linked Sex-linked disorders caused by a recessive allele have a unique pattern of inheritance A son receives his X chromosome from his mother and passes it on only to his daughters, since the gene doesn’t exist on his Y chromosome Sex-linked genes typically skip generations because the affected male passes the trait to a phenotypically normal carrier daughter, who in turn bears affected sons Several defective alleles for characteristics encoded on the X chromosome are known, including red-green color deficiency, muscular dystrophy and hemophilia Muscular dystrophy – fatal degeneration of the muscles in young boys (use the muscles, they tear, die, and are replaced by fat) Recessive It is still around because the gene is very, very long making it more susceptible to mutations.

Hemophilia Among the Royal Families of Europe
unaffected male unaffected female Edward Duke of Kent Victoria Princess of Saxe-Coburg Albert Prince of Saxe- Coburg-Gotha Queen of England Louis IV Grand Duke of Hesse-Darmstadt Alice Princess of Hesse Mary Elizabeth Alexandra Tsarina Frederick Ernest Irene Olga Tatiana Maria Anastasia Alexis Tsarevitch Edward VII King of England of Denmark Leopold Duke of Albany Helen Waldeck-Pyrmont Henry Prince of Battenburg Beatrice present British royal family (unaffected) Alexander Alfonso XII of Spain Maurice Crown Juan died in infancy Marie Jaime Gonzalo carrier daughter and hemophiliac grandson several unaffected chidren carrier female hemophiliac male Nicholas II of Russia ? Fig Recessive Deficiency of a protein needed for blood clotting Bruise easily and may bleed extensively from minor injuries.

How Do Errors in Chromosome Number Affect Humans?
The incorrect separation of chromosomes or chromatids in meiosis is known as nondisjunction Nondisjunction causes gametes to have too many and too few chromosomes Most embryos that arise from fusion of gametes with abnormal chromosome numbers spontaneously abort, but some survive to birth and beyond An individual needs to have at least one X to survive.

Some genetic disorders are caused by abnormal numbers of sex chromosomes (continued) Turner syndrome (XO) occurs in females with only one X chromosome At puberty, hormone deficiencies prevent XO females from menstruating or developing secondary sexual characteristics Hormone treatment promotes physical development, but because affected women lack mature eggs, they remain infertile More susceptible to recessive disorders such as red-green color blindness and hemophilia Additional symptoms include short stature, folds of skin around the neck, and increased risk of cardiovascular disease, kidney defects, and hearing loss 1 in 3000 females

Some genetic disorders are caused by abnormal numbers of sex chromosomes Trisomy X (XXX) results in a fertile “normal” woman with an extra X chromosome Most affected women show no abnormal symptoms There is an increased chance of learning disabilities and a tendency toward tallness By some unknown mechanism that prevents an extra X chromosome from being included in their eggs, women with trisomy X bear normal XX and XY children 1 in 1000 females

Some genetic disorders are caused by abnormal numbers of sex chromosomes Men with Klinefelter syndrome (XXY) have an extra X chromosome Most afflicted males show no symptoms, although some may show mixed secondary sexual characteristics, including partial breast development, broadening of the hips, and small testes XXY men are often infertile because of low sperm count but are not impotent 1 in 1000 Don’t find out until they are trying to have children.

Some genetic disorders are caused by abnormal numbers of sex chromosomes Males with Jacob syndrome (XYY) have an extra Y chromosome Have high levels of testosterone, tend to develop severe acne, and may be exceptionally tall, more susceptible to learning disabilities 1 in 1000 males

Some genetic disorders are caused by abnormal numbers of autosomes Nondisjunction of autosomes can occur during meiosis in the father or mother, resulting in eggs or sperm that are missing an autosome or that have two copies of an autosome Embryos with one or three copies of an autosome (trisomy) usually spontaneously abort; however, a small fraction of embryos with three copies of chromosomes 13, 18, or 21 survive to birth The frequency of nondisjunction increases with the age of the parents Frequency increases especially with the age of the mother.

Some genetic disorders are caused by abnormal numbers of autosomes In trisomy 21 (Down syndrome), afflicted individuals have three copies of chromosome 21 1 in 800 births Symptoms: weak muscle tone, small mouth with normal sized tongue, distinctively shaped eyelids heart malformations, low resistance to infectious diseases, and varying degrees of mental retardation.

Chapter 10 Patterns of Inheritance.

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Chapter 10 Patterns of Inheritance.

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