Chapter 12 Human Genetics

Family pedigree Diagram which represents immediate and extended family Standard symbols used to represent family members = affected female = affected male = mating = death = female Parents Siblings = male = heterozygotes known for sure (carriers)

Pedigree generations Roman Numerals = generations; Arabic numerals = individuals within the generation I II III 1 2

How to analyze a pedigree

Simple recessive heredity Most disorders are caused by mutations and tend to be recessive Cystic fibrosis –Thick mucus in lungs (mucus clots block airways) and digestive tract (can’t absorb sufficient food) Tay-Sachs disease (a storage disorder) –Ashkenazic Jews (from eastern Europe) –Lack normal enzyme to process lipids in brain cells Lipids fill up cells and block normal processes

Swollen lysosomes filled with lipids Tay-Sachs child

Phenylketonuria (PKU) –Inability to convert phenylalanine (an amino acid) into tyrosine (another amino acid) Phenylalanine accumulation causes mental retardation

Phenylalanine is abundant in milk and milk products Infants and toddlers are put on special diet low in phenylalanine until their brains are developed -- child can then continue life normally. WARNING LABELS: on all diet drinks which are sweetened with Aspartame (phenylalanine derivative)

Heel-prick blood test 11-year-old; strict diet

Identical twins; age 7; strict diet; gifted and talented children

PROBLEM: phenylketonuric females can enter adulthood and become pregnant (fetus could be either Pp or pp) –Mother’s high blood levels of phenylalanine can damage the fetus –What would be a solution for the mother???????

Simple dominant traits Earlobes, eyelash length, PTC taster, etc Huntington’s disease (H) –Affects motor (muscle) control between ages –Disease runs its course in years May cause shortened life depending on when symptoms begin to occur –Drug treatment causes confusion, anxiety in patients –A DNA test exists to reveal if a young person is Hh. –Question: if parents are Hh x hh, what is % chance that a child will have Huntington’s?

Complex patterns of heredity Incomplete dominance –RR x rr = Rr –(Red) x (white) = (Pink) Codominance –BB x WW = BW –(Black) x (White) = Black/White checkered feathers

Multiple phenotypes from multiple alleles –Common in nature to have more than one pair of alleles which control expression of a trait, but a single individual may only have two alleles (one from each parent) –Pigeon coloration B A = ash-red is dominant to B = blue B is dominant to b = chocolate color

–Pigeon coloration B A = ash-red is dominant to B = blue B is dominant to b = chocolate color Prob. B A b x Bb; what colors of birds could result? – B A B = ash-red – B A b = ash-red – Bb = blue – bb = chocolate

Sex-determination Discovered in fruit fly chromosomes Autosomes Sex Chromosomes Autosomes

Same method of sex determination occurs in humans –22 pairs of chromosomes = autosomes –1 pair of chromosomes = sex chromosomes XX = female (homologous pair) XY = male (nonhomologous pair)

Human karyotypes

Sex-linked inheritance Refers to an allele located on a sex chromosome (usually the X chromosome) First noticed in fruit fly eye color by T.H. Morgan

– X R = red eye allele on X –X r = white eye allele on X –X R X R = red-eyed female –X r X r = white-eyed female –X R Y = ??? –X r Y = ??? –Proportionally, more male flies in nature are white-eyed than females -- why might this be?

Which symbols should be used for each fly? XRXR XrXr Y

Prob. What fruit fly offspring are produced when one crosses X R X r x X R Y XRXR XrXr XRXR Y

Polygenic inheritance Single trait produced by several pairs of genes –Skin color, height, and others –Graph of all expressions vs. quantities is bell-shaped

Polygenic example –AABBCCDDEEFF = most darkly pigmented black person (probably an equatorial African); maximum melanin – aabbccddeeff = most lightly pigmented person (probably Finnish/Swedish, etc.); minimum melanin What would AABBCCDDEEFF x aabbccddeeff look like? AaBbCcDdEeFf = medium brown

Environmental factors which affect phenotypes External factors –Temperature: notice effect on Serratia marcescens (bacterial species), p. 327 top Room temp. = forms red pigment Temp. >30ºC = no pigment (white) –Leaf size: depends on amount of light received Internal factors –Sex-influence Lion’s mane, beard growth on men, ram’s horns, male pattern baldness, bird plumage (usually more colorful on males)

Normal Red Blood CellsSickled Red Blood Cells

Codominance in humans Red blood cell pigment = hemoglobin (red) –H A = normal hemoglobin allele –H S = sickled hemoglobin allele (RBCs don’t attract O 2 ; cells stick together easily and make clots); cells don’t last 120 days like normal cells -- so fewer RBCs = anemia Normal human genotype = H A H A H A H S = sickle cell trait (individual is somewhat anemic) –Each allele is codominant H S H S = sickle cell disease (very anemic; requires transfusions periodically)

Human multiple alleles Human blood phenotypes: A, B, AB, O –3 alleles in human gene pool I A, I B, i Each person inherits only 2 alleles to make the blood type. Alleles code for surface polysaccharides to be placed on RBC’s membranes.

What “makes” actual blood types?

B Pedigree of blood types; ? O O A ? OAB Blood type? Genotypes

Problems: 1.Mother is blood type O, father is blood type A; what blood type(s) might a child have? 2. Mother is blood type AB, child is blood type A; mother claims that Charles (blood type O) is the father -- is he? 3. Mother is blood type O, child is blood type B; mother claims that Robert (blood type AB) is the father -- is he?

Sex-linked (X) traits in humans Red-green color blindness –X C = normal color vision –X c = color blindness –What is the genotype of a color blind male? –… a color blind female? Hemophilia: bleeder’s disease –X H = normal blood clotting –X h = lack of blood clotting (individuals missing Factor VIII factors needed to complete normal blood clotting) –If you were a child of a female carrier, what would be your chances of having (carrying) the trait?

Y-linked trait: hairy pinna (allele is on Y chromosome)

Unusual chromosome numbers Usually caused by nondisjunction –Monosomy X = Turner’s syndrome Slender, underdeveloped, sterile females, varying mental retardation –Klinefelter’s syndrome (trisomy of sex chromosomes: either XXY or XYY); can be any number of X chromosomes, but just one Y makes individual a male. –Ex. XXXY, XXXXXY Tall, slender, underdeveloped, sterile males, varying mental retardation –Trisomy X = metafemale –Trisomy 21 = Down’s syndrome Chromosomes are pictured in descending sizes in a karyotype in order to see monosomies and trisomies

Human karyotypes

Review Questions

Easy Questions (if you studied)

Question The 23rd pair of chromosomes that differ in males and females are called _____. a.Autosomes b.Sex chromosomes c.Multiple alleles d.Polygenes

Question If a female fruit fly heterozygous for red eyes (X R X r ) crossed with a white- eyed male (X r Y), what percent of their offspring would have white eyes? a.0% b.25% c.50% d.75%

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