Color blindness is a sexlinked trait in humans

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

Figure 10. 15 a, b Linkage hypothesis White eyes Gray body w w + + Gametes y y Morgan’s test of the hypothesis w y+ + + w w w y+ y Parental generation Red eyes Yellow body y Meiosis I w y+ y F 2 generation males wy+ w+ y + w+y / Y Male wy / wy Female X wy+/ Y wy+/ w+y Meiosis II w y+ Male X + F 1 generation w+ w+ y Female w+ y w+y Hypothesis: When two loci occur on one chromosome, meiosis results in two, rather than four, types of gametes because independent assortment does not occur. Phenotype Genotype Number wy+ /Y + w y /Y Novel genotypes 4292 4605 wy /Y 86 w+y+ /Y 44 Results: There are four kinds of gametes rather than two due to occurrence of two recombinant genotypes.

Figure 10. 15 c Crossing over provides an explanation w y+ w w y+ y w+ w+ y+ y Crossing over during meiosis I w+ w+ y+ y Gametes Meiosis II w w W+ w+ y+ y wy+ wy w+y+ Recombinant chromosomes w+y

Figure 10. 16 Gene 1 Gene 2 Gene 3 Gene 4 Gene 5 Gene 6 Gene 7 Gene 8 Gene 9 Gene 10 Gene 11 Gene 12 Chromosomes are composed of genes Crossing over rarely occurs between adjacent loci, so recombinations are rare. 0 : Yellow body 1. 4 : White eyes Map units 20 : Cut wings Crossing over almost always occurs between distant loci, so recombinations are frequent. The physical distance between loci determines the frequency of crossing over. 0 : Yellow body 1. 4 : White eyes Frequency of crossing can be used to map physical between loci. Linkage map