Fig

I. I.Linkage and Recombination B. 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

II. II.Sex Chromosomes and Gender Many systems of sex determination Most animals have sex chromosomes One gender typically homogametic, the other heterogametic A. 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

II. II.Sex Chromosomes and Gender A. 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) 1)X chromosome required for development 2) 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)

II. II.Sex Chromosomes and Gender B. B.Other Species 1. 1.X-Y system XX = Female, XY = Male Used by humans 2. 2.X-O system 3. 3.Z-W system 4. 4.Haplo-diploid system

1. 1.X-Y system XX = Female, XY = Male Used by humans 2. 2.X-O system (crickets) Egg carries X Sperm carries X or nothing 3. 3.Z-W system (birds, fishes, butterflies, moths) Egg carries Z or W Sperm carries Z 4. 4.Haplo-diploid system (ants) Females from fertilized ova Males from unfertilized ova Fig. 15.6

II. II.Sex Chromosomes and Gender C. 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 P: Wild type female, red eyes Mutant male, white eyes F 1 : All with red eyes F 2 : Females with red eyes Half of males with white eyes

II. II.Sex Chromosomes and Gender C. 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

II. II.Sex Chromosomes and Gender C. C.Sex-Linked Genes Disorders 1) 1)Color blindness Most common in men 2) 2)Duchenne muscular dystrophy Absence of key muscle protein (dystrophin) 3) 3)Hemophilia Absence of protein(s) required for blood clotting

II. II.Sex Chromosomes and Gender D. 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

II. II.Sex Chromosomes and Gender E. 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 B 1 = Pattern baldness, B 2 = Normal hair growth B 1 B 1 = Pattern baldness in males & females B 1 B 2 = Pattern baldness in males, normal hair in females B 2 B 2 = Normal hair in males & females

III. III.Chromosomal Abnormalities A. 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

III. III.Chromosomal Abnormalities A. A.Chromosome Number 1. 1.Possible outcomes a. a.Trisomy – 2n+1 chromosomes in fertilized egg b. b.Monosomy – 2n-1 chromosomes in fertilized egg c. c.Polyploidy – 3n, 4n, 5n, 6n, etc. chromosomes in fertilized egg Triploidy – 3n chromosomes Tetraploidy – 4n chromosomes

III. III.Chromosomal Abnormalities A. A.Chromosome Number 2. 2.Possible outcomes Autosomal aneuploidies highly detrimental and rare No known autosomal monosomies (100% lethal) a. 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

III. III.Chromosomal Abnormalities A. A.Chromosome Number 2. 2.Possible outcomes Sex chromosome aneuploidies less rare, perhaps due to dosage compensation and few genes on Y b. 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. c.XYY Phenotypically male but often very tall May have severe acne d. d.XXX Phenotypically normal female e. e.Turner Syndrome (XO) Phenotypically female with no Barr bodies Usually with undeveloped reproductive structures

III. III.Chromosomal Abnormalities B. B.Chromosome Structure Often results from breakage of chromosomes and errors in repair

Fig

III. III.Chromosomal Abnormalities B. B.Chromosome Structure 1. 1.Deletion Cri du Chat Syndrome Deletion of part of short arm of chromosome 5 Mental retardation, small head, cry like a kitten 2. 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

IV. IV.Exceptions to Mendelian Inheritance A. 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