fig. 15.11. i. i.linkage and recombination b. b.recombination possible to use recombination...

Fig. 15.11

I. Linkage and Recombination

B. Recombination• Possible to use recombination frequencies to

construct genetic map (linkage map) of genes on chromosome

• If two loci are sufficiently far apart, possible to get double crossing over

Fig. 15.12

II. Sex Chromosomes and Gender

• Many systems of sex determination• Most animals have sex chromosomes• One gender typically homogametic, the other

heterogametic

A. Humans• Males and females with 22 pairs of autosomes

(do not determine gender)• Question: Is female gender in humans

determined by presence of two X chromosomes or absence of Y chromosome?

• Approach: Examine people with abnormal sex chromosomes


A. Humans• XXY – Klinefelter Syndrome

• Nearly normal males with underdeveloped testes

• XO – Turner Syndrome• Phenotypically female with underdeveloped ovaries

• YO – Embryo doesn’t develop• Conclusions

1) X chromosome required for development

2) Genes on Y chromosome determine gender

• X and Y chromosome aren’t homologous but have short, homologous pairing regions that permit synapsis during meiosis

• Sperm containing X and Y chromosomes produced in equal numbers

• More male babies conceived, die before birth, born (1.06:1)


B. Other Species1. X-Y system

• XX = Female, XY = Male• Used by humans

2. X-O system

3. Z-W system

4. Haplo-diploid system

1. X-Y system• XX = Female, XY = Male• Used by humans

2. X-O system (crickets)• Egg carries X• Sperm carries X or nothing

3. Z-W system (birds, fishes, butterflies, moths)

• Egg carries Z or W• Sperm carries Z

4. Haplo-diploid system (ants)• Females from fertilized ova• Males from unfertilized ova

Fig. 15.6


C. Sex-Linked Genes• X chromosome in humans contains many genes

required by both males and females• Y chromosome contains fewer genes, mostly

related to “maleness” (testicular development, affinity for monster trucks, etc.)

• Mutations on X chromosome can lead to genetic disorders (X-linked)

Fig. 15.4

• P: Wild type female, red eyes• Mutant male, white eyes

• F1: All with red eyes

• F2: Females with red eyes

• Half of males with white eyes


C. Sex-Linked Genes• Females

• Inherit one X from mother and one from father• Dominant traits expressed, recessive traits not• Heterozygous – Can be carriers

• Males• Inherit all X-linked genes from mother• All X-linked alleles typically expressed• Hemizygous – Can’t be carriers

Fig. 15.7


C. Sex-Linked Genes• Disorders

1) Color blindness• Most common in men

2) Duchenne muscular dystrophy• Absence of key muscle protein (dystrophin)

3) Hemophilia• Absence of protein(s) required for blood clotting


D. X Inactivation• Females have two copies of X chromosome

• Fruit flies – Males make single X more active than either female X

• Mammals – One X typically inactivated at random in each cell (dosage compensation)• Barr body – Inactivated X, visible during

interphase as dark area of highly condensed chromatin

• Inactivation incomplete; some genes expressed• Heterozygous female may express traits from

each X chromosome in ~50% of cells• Ex: Calico and tortoiseshell cats

Fig. 15.8


E. Sex-Influenced Genes• Some traits inherited autosomally but influenced

by gender• Male & female with same genotype, different

phenotypes

• Ex: Pattern baldness• Proposed that single pair of alleles determines pattern

baldness – Dominant in males, recessive in females• B1 = Pattern baldness, B2 = Normal hair growth

• B1B1 = Pattern baldness in males & females

• B1B2 = Pattern baldness in males, normal hair in females

• B2B2 = Normal hair in males & females

III. Chromosomal Abnormalities

A. Chromosome Number• Usually due to nondisjunction (chromosomes

fail to separate during anaphase of meiosis)• One gamete receives an extra chromosome, the

other receives one fewer than normal• Condition = aneuploidy• Nondisjunction during mitosis may lead to clonal

cell lines with abnormal chromosome counts• Nondisjunction during meiosis may lead to

gametes (and offspring) with abnormal chromosome counts

Fig. 15.13


A. Chromosome Number1. Possible outcomes

a. Trisomy – 2n+1 chromosomes in fertilized egg

b. Monosomy – 2n-1 chromosomes in fertilized egg

c. Polyploidy – 3n, 4n, 5n, 6n, etc. chromosomes in fertilized egg• Triploidy – 3n chromosomes• Tetraploidy – 4n chromosomes



• Autosomal aneuploidies highly detrimental and rare• No known autosomal monosomies (100% lethal)

a. Down’s Syndrome• Trisomy of chromosome 21• Mental retardation, heart defects, susceptibility to

diseases• Affects ca. 1 of every 700 children born in US• Frequency increases with age of mother

Fig. 15.15



• Sex chromosome aneuploidies less rare, perhaps due to dosage compensation and few genes on Y

b. Klinefelter Syndrome (XXY)• Phenotypically male but with Barr bodies• Tend to be tall with female-like breasts and reduced

testes• May show signs of mental retardation

c. XYY• Phenotypically male but often very tall• May have severe acne

d. XXX• Phenotypically normal female

e. Turner Syndrome (XO)• Phenotypically female with no Barr bodies• Usually with undeveloped reproductive structures


B. Chromosome Structure• Often results from breakage of chromosomes

and errors in repair

Fig. 15.14


B. Chromosome Structure1. Deletion

• Cri du Chat Syndrome• Deletion of part of short arm of chromosome 5• Mental retardation, small head, cry like a kitten

2. Translocation• Down’s syndrome may be caused not by trisomy but

by extra material from chromosome 21 attached to other, large chromosome

• Reciprocal translocation between chromosomes 9 and 22 can increase likelihood of developing chronic myelogenous leukemia (CML)

Fig. 15.16

IV. Exceptions to Mendelian Inheritance

A. Genomic Imprinting• Expression of phenotype affected differently by

inheritance of allele from mother vs. father• Imprinting may lead to expression of maternal or

paternal allele for a particular species and gene

Fig. 15.17

fig. 15.11. i. i.linkage and recombination b. b.recombination possible to use recombination...

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