Color blindness is a sex-linked trait in humans In humans, the X chromosome has about 5,000 genes while the Y chromosome has only 30 genes –Females are XX, males are XY –There is no locus on the Y chromosome for the color vision/color blindness trait –Color blindness is far more prevalent in males because there is not a second allele to mask the recessive allele that causes color blindness

Hemophilia

Which of a man's grandparents could not be the source of any of the genes on his Y-chromosome? A. Father's Mother. B. Mother's Father. C. Father's Father. D. Mother's Mother, Mother's Father, and Father's Mother. E. Mother's Mother.

Pedigree analysis

Nondisjunction

Turner’s Syndrome

Klinefelter Syndrome

Sex determination in Drosophila Although males have an X and a Y chromosome, the Y chromosome does not determine the sex of a fruit fly Rather, the number of X chromosomes determines the sex –Fruitflies with two X chromosomes become female –Fruitflies with one X chromosome become male –Through nondisjunction, a fruitfly with only an X chromosome but no Y will become male; a fruitfly with two X chromosomes and a Y chromosome will become female

Sex Determination in Birds Female is heterogametic sex

Sex determination in honeybees Diploid individuals are females Haploid individuals are males

In some reptiles, sex determination is environmental Some species of turtles –Above 32 o C, females produced –Below 28 o C, males produced –Between 28 and 32 o C, mixture of males and females However, snapping turtle eggs incubated either below 20 o C or above 30 o C are female; males produced predominantly in between temperatures Note that although environmental factors trigger the sexual developmental pathway, the pathways themselves are under genetic control

Parental generation F 1 generation F 2 generation males Meiosis I Meiosis II w w w+w+ w+w+ y+y+ y+y+ yy PhenotypeGenotypeNumber FemaleMale FemaleMale X X Hypothesis: When two loci occur on one chromosome, meiosis results in two, rather than four, types of gametes because independent assortment does not occur. Results: There are four kinds of gametes rather than two due to occurrence of two recombinant genotypes. Novel genotypes w+w+ w+w+ w+w+ w+w+ w w ww y y yy y+y+ y+y+ y+y+ y+y+ wy + /Y w + y /Y wy /Y w + y + /Y wy + / w + y wy + / Y w + y / Y wy + / wy + w+yw+y wy + Gametes Red eyes Yellow bodyGray body White eyes Figure 10.15a,b Linkage hypothesisMorgan’s test of the hypothesis

www+w+ w+w+ y+y+ yy+y+ y w+w+ w+w+ ww y+y+ yyy+y+ Crossing over during meiosis I Meiosis II W + w+w+ w w y+y+ y+y+ yy wy + wyw+y+w+y+ w+yw+y Recombinant chromosomes Gametes Figure 10.15c Crossing over provides an explanation

Gene 1 Gene 2 Gene 3 Gene 4 Gene 5 Gene 6 Gene 7 Gene 8 Gene 9 Gene 10 Gene 11 Gene 12 Crossing over rarely occurs between adjacent loci, so recombinations are rare. Crossing over almost always occurs between distant loci, so recombinations are frequent. 0 : Yellow body 1.4 : White eyes Map units 0 : Yellow body 1.4 : White eyes 20 : Cut wings Figure Chromosomes are composed of genes The physical distance between loci determines the frequency of crossing over. Frequency of crossing can be used to map physical between loci. Linkage map