Chromosomal Basis of Heredity

1. Chromosomal Basis of Heredity Studying Inheritance Patterns for Genes on Specific Chromosomes

2. Sex Determination Sex Chromosomes: homologous chromosomes that differ in size and genetic composition between males and females

3. Autosomes Autosome: any chromosome that is not a sex chromosome Humans have 22 pairs of autosomes and 1 pair of sex chromosomes

4. X1 X2 Female Parent EGGS X1 X1 X1 X2 X2 X2 SPERM Female Offspring Male Parent Xm Xm Xm Xm Y Y Y Y Male Offspring Sex Determinationin Mammals Expected Ratio½ female: ½ male

5. R r XRY XrY XRXR XRXr R Genotypes Frequencies Phenotypes Frequencies R = redr = white Sex Linked Genes: Eye Color in Fruit Flies Eggs of XR XrFemale XR XR XR Xr Sperm ofXRY Male Female Female Y XR Y Xr Male Male 1 1 1 1 White male Red female Red female Red male 25% 25% 25% 25%

6. Females can be homozygous or heterozygous for traits on the X chromosome Males carry only one copy of genes on the X chromosome H = no hemophilia h = hemophilia Sex-linked Genes in Humans GrampsXhY Hemophilic Male Non-hemophilic Female (father is hemophilic) XhY x XHXh XH Xh XHXh XhXh Xh Y XHY XhY Phenotypic Ratio of Offspring ¼ hemophilic males + ¼ non-hemophilic males ¼ hemophilic females + ¼ non-hemophilic females

7. Normal Meiosis Non-disjunction in Meiosis I Non-disjunction in Meiosis II Human Chromosomal Abnormalities Arise by Non-disjunction = Failure of homologues or chromatids to separate during meiosis

8. Human Autosomal Abnormality How can Down Syndrome occur? Eg. Egg with 2 copies of #21 (24 chromosomes) + Sperm with 1 copy of #21 (23 chromosomes) = Embryo with 3 copies of #21 (47 chromosomes)

9. 400 300 200 100 0 10 20 30 40 50 Incidence of Down Syndrome Number per 1000 Births Age of Mother (years)

10. Karyotype for Down Syndrome Physical Features Eye fold Palm Crease

11. Human Sex Chromosome Abnormality How can Turner Syndrome occur? Eg. Egg with 0 copies of X (22 chromosomes) +Sperm with 1 copy of X (23 chromosomes)= Embryo with 1 copy of X (45 chromosomes)

12. Karyotype for Turner’s Syndrome Non-functional Ovaries From Adult Female with Turner’s Syndrome Normal uterus, tubes and ovaries