Human Heredity

11.3

Human Chromosomes

Humans have 46 chromosomesOccurs in 23 pairs 22 pairs are autosomes2 of the 46 are sex-determining chromosomes (X and Y)XX=Female, XY=MalePicture of chromosomes=karyotypekaryotypeDoctors use to determine sex of baby and genetic abnormalities

Karyotype Studies

Karyotype—micrograph in which the pairs of homologous chromosomes are arranged in decreasing size.

Images of chromosomes stained during metaphase

SAMPLE KARYOTYPE

Nondisjunction

Cell division during which sister chromatids fail to separate properly

Example: Down syndrome – person has an extra copy of chromosome 21

Pedigree

Chart that shows relationships within families often used to trace lineage of a disease/disorderCircle=FemaleSquare=MaleShaded=Affected__ marriageI children

Human Genes

Autosomal Recessive Disorders-caused by 2 recessive alleles present (ex: PKU, Tay Sach’s, CF)

Autosomal Dominant Disorders-only one allele present causes affliction (ex: dwarfism, Huntington’s Disease)

Codominant Alleles-Sickle Cell

Recessive Disorders

A.Cystic fibrosis: Homozygous recessives (cc) have cystic fibrosis - body cannot make needed chloride channel, high concentrations of extracellular chloride causes mucous to build up, infections, pneumonia. Diet, antibiotics and treatment can extend life to 25 years or more.B.Tay-Sachs: Enzyme that breaks down brain lipids is non-functional in homozygous recessives (tt). Buildup of lipids causes death by age 2-3

Dominant Disorders*Achondroplasia (dwarfism): AA =

Homozygous dominant is lethal - fatal (spontaneous abortion of fetus). Aa = dwarfism. aa = no dwarfism. 99.96% of all people in the world are homozygous recessive (aa)

Dominant Disorders

B. Polydactyly (extra fingers or toes): PP or Pp = extra digits, aa = 5 digits. 98% of all people in the world are homozygous recessive (pp)

Co Dominant Alleles

ss= sickle cell anemia (misshapen Sickle Cell: many believe sickle cell anemia co evolved with malariacells lead to death)SS= normal red cells, healthySs= not enough sickle cells to kill and resistant to malaria! (body destroys the sickle cells and the parasite)