gen bio - lecture 18: human genetics

why is it difficult to study genetic variation in humans

1. few offspring

2. long generation time (20-30 years)

3. unethical

pedigree

shows inheritance pattern when a trait is controlled @ a single locus

mendelian-inheritance patterns

• explains how single genes produce predictable offspring ratios

  • single gene traits

  • predictable ratios

  • clear dominance relationships (dominant OR recessive)

autosomal recessive (in relation to ex. sickle cell disease) OVERVIEW

• dominant allele makes enough protein alone SO to express a disorder, one must be HOMOZYGOUS RECESSIVE

sickle cell disease outcomes (homozygous recessive, heterozygotes, heterozygote advantage)

• homozygous recessive (have sickle cell anemia)

  • regular cells are destroyed immediately

    • they have a lower O2 carrying capacity = anemia

  • sickle cells all clump together

    • form blood clots, slow blood flow, block capillaries

    • leads to tissue damage and pain

• heterozygotes/carriers = CODOMINANT (both alleles expressed equally)

  • normal cells produce hemoglobin; abnormal cells do not

  • often NO PHENOTYPIC difference in heterozygous and homozygous dominant individuals

• heterozygote advantage!

  • malaria (intercellular parasite of RBCs) and heterozygotes are resistant!!

  • sickle cell allele favored in regions with endemic malaria

autosomal dominant

• only one allele required

• phenotypes often only apparent in life AFTER reproduction

example: condition of having 6 fingers instead of 5

• has a dominant allele; means heterozygotes also affected

non-mendelian inheritance patterns (when categorizes them as such)

• traits deviate from mendel’s laws when genes are on the same chromosome or influenced by sex

  • sex linked inheritance

  • gene linkage

  • crossing over

disomy

2n

• normal for humans

nondisjunction

when mistakes occur and homologous chromosomes or sister chromatids fail to separate

aneuploidy (include trisomy and monosomy) + viabilities

• aneuploidy: presence of an abnormal # of a particular chromosome

• trisomy: 3 copies of one particular chromosome (2n+1) → down syndrome

  • most trisomy = inviable

• monosomy: missing 1 member of a pair of chromosomes (2n-1)

  • monosomy for autosomes = die in utero!

aneuploidy of sex chromosomes (turner syndrome, klinefelter syndrome, xyy males)

• many sex chromosome aneuploidies are viable BUT infertile!

• turner syndrome (XO)

  • 44 autosomes and 1 x chromosome

  • denoted as XO or 45, XO

  • common symptoms:

    • short stature

    • webbed neck

    • sterile

    • no barr bodies

• klinefelter syndrome (XXY)

  • 44 autosomes, 2x, 1y

  • denoted XXY or 47, XXY

  • common symptoms:

    • tall stature

    • often learning disability

• XYY males

  • 44 autosomes, 1x, 2y

  • no specific phenotype

  • usually fertile but do not transmit extra y

  • “supermales”

changes to chromosome structure (why they occur + diff types)

• occur due to errors in meiosis, radiation, or other mutagens

• include…

  • deletion (removes a chromosomal segment)

  • duplication (repeats a segment)

  • inversion (reverses a segment within a chromosome)

  • translocation (moves a segment from one chromosome to a nonhomologous chromosome)

fetal testing

• genetic test done before birth

• earlier diagnosis = better chance of prevention/alleviation

• methods:

  • amniocentesis

  • chronic villus sampling

  • non-invase prenatal screening/testing

newborn screening (including phenylketonuria PKU + how to manage)

• blood prick + smear (24-48 hrs) can help detect some genetic disorders

• phenylketonuria PKU

  • every newborn in US is tested

  • normally phenylalanine leads to tyrosine (not toxic) but PKU creates phenylketone (TOXIC)

• managing PKU

  • low phenylaline diet can prevent symptoms

  • generally cannot eat meat, fish, dairy nuts, bread, soda