Human Genetics

Genetics of sex Women & men are very different, but just a few genes create that difference In mammals = 2 sex chromosomes –X & Y –2 X chromosomes = female: XX –X & Y chromosome = male: XY XY XX

Sex-Linked Traits X chromosome Y chromosome Boy or Girl? The Y Chromosome “determines” The Y chromosome of human males is only about one-third the size of the X chromosome

Sex-linked traits Sex chromosomes have other genes on them, too –especially the X chromosome –hemophilia in humans blood doesn’t clot –Duchenne muscular dystrophy in humans loss of muscle control –red-green color blindness see green & red as shades of grey XY XX

Sex-linked characteristics are found only on the X chromosome and not on the Y because Y is too small to carry these genes X B X B – Normal Female X B X b – Carrier Female (do NOT show disorder) X b X b – Diseased Female X B Y - Normal Male X b Y - Diseased Male Males only get 1 copy of the gene Sex-linked disorders are more common in males than females. Females must get both recessives genes to have the disorder while it only takes 1 for males

Sex-Linked Traits Carriers can pass the gene, but do not exhibit the symptoms of disorders X B Y x X B X b X B Y x X B X b NormalCarrier Male Female

Sex-linked traits XHXH Y male / sperm XHXH XhXh female / eggs XHXHXHXH XHXhXHXh XHYXHYXhYXhY XHXhXHXh XHXH XhXh XHYXHY Y XHXH XHXHXHXH XHYXHY XHXhXHXh XhYXhY 2 normal parents, but mother is carrier HH Hh x XHYXHY XHXhXHXh

Sex-Linked Traits Fathers= pass X-linked alleles to all daughters only (but not to sons) Mothers= pass X-linked alleles to both sons & daughters

Multiple Alleles in Human Blood There are 3 types of alleles (genes) in human blood I A I B I O or i A & B alleles are codominant both A & B alleles are dominant over i allele We only have 2 of these three genes at a time, one from each parent

I A and I B are equally dominant and I O is recessive GenotypePhenotype I A I A I A i Type A I B I B I B i Type B I A I B Type AB ii Type O Co-dominance in Human Blood

IAIA IBIB IBIB IAIA i i ii IAIA i IBIB i Co-dominance Type A x Type B phenotypic ratio = 1:1:1:1 1 type AB 1 type A 1 type B 1 type O

Many genes: one trait Polygenic inheritance –additive effects of many genes –humans skin color height weight eye color intelligence behaviors

Albinism albino Africans melanin = universal brown color

Dominant ≠ most common allele Polydactyly dominant allele Because an allele is dominant does NOT mean… –it is better, or –it is more common recessive allele far more common than dominant  only 1 individual out of 500 has more than 5 fingers/toes  so 499 out of 500 people are homozygous recessive (aa)

Genetic Disorder Huntington’s disease (HD) is an inherited disorder caused by the degeneration of nerve cells in the brain. HD causes involuntary movements and loss of intellectual abilities (dementia).

Chromosomal Errors NONDISJUNCTION: the failure of chromosomes to separate properly in meiosis. Gametes with extra or too few chromosomes result.

Genetic Disorder Fragile X Syndrome: A disorder caused by a defective gene on the X-chromosome.

Genetic Disorder Symptoms of Fragile X Syndrome: –mental retardation, –Enlarged testes, –facial abnormalities in males –mild or no effects in females. It is the most common inherited cause of mental retardation.

Genetic Disorder Cri-du-chat Syndrome is a rare genetic disorder due to a missing portion of chromosome # 5. Its name, meaning cat cry in French, is from the distinctive mewing sound made by infants with the disorder.

Genetic Disorders Cri-du-chat Syndrome is characterized by: –distinctive facial features, –small head size, –low birth weight, –weak muscle tone, –a round face, –low set ears, –facial asymmetry –severe mental retardation is typical

Phenotype is controlled by both environment & genes Color of Hydrangea flowers is influenced by soil pH Human skin color is influenced by both genetics & environmental conditions Environment effect on genes Coat color in arctic fox influenced by heat sensitive alleles

Pedigree Pedigree charts are used to study the transmission of a hereditary condition.