Chromosomal Basis of Heredity

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Chromosomal Basis of Heredity
Studying Inheritance Patterns for Genes on
Specific Chromosomes
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Sex Determination

• Sex Chromosomes homologous chromosomes that
  differ in size and genetic composition between
  males and females

Human Chromosome X Y
Size Larger Smaller
Genetic Composition Multiple genes unrelated to gender Very few genes TDF region determines maleness
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Autosomes

• Autosome any chromosome that is not a sex
  chromosome
• Humans have 22 pairs of autosomes and 1 pair
  of sex chromosomes

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Sex Determinationin Mammals

• Expected Ratio½ female ½ male

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Sex Linked Genes Eye Color in Fruit Flies
R redr white
Eggs of XR Xr Female
Sperm ofXRY Male
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Sex-linked Genes in Humans
H no hemophilia h hemophilia
Hemophilic Male Non-hemophilic
Female
(father is hemophilic)
Phenotypic Ratio of Offspring
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Human Chromosomal Abnormalities
Arise by Non-disjunction Failure of homologues
or chromatids to separate during meiosis
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Human Autosomal Abnormality
Down Syndrome Trisomy 21Three copies of chromosome 21
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)
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Incidence of Down Syndrome
Number per 1000 Births
Age of Mother (years)
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Karyotype for Down Syndrome
Physical Features
Eye fold
Palm Crease
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Human Sex Chromosome Abnormality
Turner Syndrome XO One copy of X No second sex chromosome
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)
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Karyotype for Turners Syndrome
Non-functional Ovaries From Adult Female with
Turners Syndrome
Normal uterus, tubes and ovaries