UNIT VI MENDELIAN GENETICS Baby Campbell Ch 9

UNIT VI - MENDELIAN GENETICS Baby Campbell – Ch 9 Big Campbell – Ch 14, 15 Hillis – Chp 8

I. MENDEL • Mendel’s Experiments pea plants o Worked with _______ o Eliminated self-pollination ____________ and controlled __________ cross-pollination o P Generation - True-breeding pea plants with one trait X true-breeding pea plants with another trait o Produced hybrids also known as F 1 TT x tt Ø ______________ Ø F 1 Phenotype = 4 tall : 0 short Ø F 1 Genotype = 0 TT : 4 Tt : 0 tt o F 1 X F 1 → F 2 Tt x Tt Ø ______________ Ø F 2 Phenotype Ratio = 3 tall : 1 short Ø F 2 Genotype Ratio = 1 TT : 2 Tt : 1 tt

I. MENDEL, cont • Mendel’s Principles 1) 2) 3) 4) 5) Alternative versions of genes known alleles as _______ account for variations in inherited characters. 2 Organisms inherit _____ alleles for each trait If alleles at a locus differ; that is; if heterozygous the genotype is ________, the allele that shows is known as the dominant ________ allele. Law of Segregation – two alleles for a heritable character segregate during meiosis Law of Independent Assortment – each pair of alleles segregates independently of each other pair of alleles during meiosis

Independent Assortment or Dependent Assortment? ? ? P generation = YYRR x yyrr Gametes = YR x yr • F 1 Generation = Yy. Rr • Predict F 2 offspring… If Yy. Rr x Yy. Rr…. • If dependent assortment… then gametes = YR yr • If Independent Assortment… Gametes are ______

II. ANALYZING PROBABILITY OF TRAIT INHERITANCE • Test Cross o Organisms with dominant phenotype (PP or Pp) crossed with homozygous recessive ___________(pp)__ to determine genotype • Punnett Square – tools used to predict the possible outcomes of meiosis and fertilization. • Multiplication Rule o States the probability of 2 or more independent events occurring together can be calculated by multiplying individual probabilities o For example, Ø Determine the probability of a homozygous recessive short plant produced from F 1 X F 1 § § Cross = Tt x Tt Probability of egg carrying t = ½ Probability of sperm carrying t = ½ Probability of tt offspring = ¼

II. ANALYZING PROBABILITIES, cont • Addition Rule o States that the probability of 2 or more mutually exclusive events occurring can be calculated by adding together their individual probabilities o For example, Ø Determine the probability of a heterozygous plant produced from F 1 X F 1 § Tt x Tt § Chance of egg carrying T = ½ § Chance of sperm carrying t = ½ § Chance of sperm carrying T = ½ § Chance of egg carrying t = ½ § Probability of Tt offspring = ¼ + ¼ = ½

II. ANALYZING PROBABILITIES, cont • Crosses Involving Multiple Characters o Determine the genotype ratios of the offspring for the cross Bb. CCDD X BBcc. Dd

II. ANALYZING PROBABILITIES, cont • Crosses Involving Multiple Characters o Determine the phenotype ratios of the offspring for the cross Yy. Rr. SS X yy. Rrss

II. ANALYZING PROBABILITIES, cont • Crosses Involving Multiple Characters o In the cross, Pp. Yy. Rr X Ppyyrr, what is the probability of offspring that are purple, green, & round? Ø P= purple, p = white Ø Y = yellow, y = green Ø R = round, r = wrinkled

Probability Practice Makes Perfect! In pea plants, Ø long stems are dominant to short stems Ø purple flowers are dominant to white, and Ø round seeds are dominant to wrinkled. § A plant that is heterozygous for all three loci self-pollinates and 2048 progeny are examined. How many of the resulting plants would you expect to be long-stemmed with purple flowers, producing wrinkled seeds?

III. VARIATIONS IN INHERITANCE • Co-Dominance o Both alleles affect phenotype in separate & distinguishable ways o Often designated with 2 different “big letters” For example, __In chickens, BB = black WW = white

III. VARIATIONS IN INHERITANCE • Incomplete Dominance o Neither allele is dominant; heterozygotes show a blend of two homozygous phenotypes o One allele designated with “big letter’, the other with “big letter prime”; for example T T’ For example, in snapdragons, RR = RED R’R’ = WHITE

III. VARIATIONS IN INHERITANCE, cont • Multiple Alleles o Many genes have more than 2 alleles o Example, ABO blood groups in humans o Three alleles Ø Ø Ø • Rh Factor • A woman with O positive blood has a child with Type A negative blood. The man she claims is the father has AB positive blood. Is it possible that he is the father of this child? Phenotype A B AB O Genotype

III. VARIATIONS IN INHERITANCE, cont • Polygenic Inheritance o For example, AABBCC = very dark skin; aabbcc = very light skin. o Intensity based on units; in other words, Aa. Bb. Cc and AABbcc individuals would have the same pigmentation

III. VARIATIONS IN INHERITANCE, cont • Epistasis o Gene at one locus alters phenotypic expression of a gene at a second locus o For example, A dominant allele, P causes the production of purple pigment; pp individuals are white. A dominant allele C is also required for color production; cc individuals are white. What proportion of offspring will be purple from a pp. Cc x Pp. Cc cross?

III. VARIATIONS IN INHERITANCE, cont • Pleiotropy – gene can have multiple phenotypic effects. EX: the gene for flower color also affects the gene for seed coat color.

III. VARIATIONS IN INHERITANCE, cont • Environmental Impact on Phenotypes • Phenotype of a trait depends on environment as well as genotype. • EX: size and shape of leaves on a tree also depends on sun, wind, water, etc. in addition to alleles. • EX: sun on skin, exercise on build, nutrition on height, etc.

IV. SEX-LINKED INHERITANCE o First recognized by Thomas Hunt Morgan Ø Drosophila melanogaster v. Fruit flies v. Excellent organism for genetic studies § Prolific breeding habits § Simple genetic make-up; 4 pairs of chromosomes → 3 pairs of autosomes, 1 pair of sex chromosomes Ø Crossed true-breeding wild-type females with true-breeding mutant males Ø Mutant trait showed up in ½ male F 2 offspring ; was not seen in F 2 females o Determined mutant allele was on X-chromosome; thus inherited differently in males versus females Ø In females, Ø In males,

IV. SEX-LINKED INHERITANCE, cont • Red-green colorblindness is caused by a sex-linked recessive allele. A colorblind man marries a woman with normal vision whose father was colorblind. What is the probability she will have a colorblind daughter?

IV. SEX-LINKED INHERITANCE, cont • The gene for amber body color in Drosophila is sex-linked recessive. The dominant allele produces wild type body color. The gene for black eyes is autosomal recessive; the wild type red eyes are dominant. If males with amber bodies, heterozygous for eye color are crossed with females heterozygous for eye color and body color, calculate the expected phenotype ratios in the offspring.

IV. SEX-LINKED INHERITANCE, cont • X Inactivation in Females o During embryonic development, one X chromosome in female cells is inactivated due to addition of methyl group to its DNA o Dosage compensation o Inactive X chromosome condenses; known as Barr body

IV. SEX-LINKED IN INHERITANCE, cont o Occurs randomly ØFemales will have some cells where “Dad’s copy” of X is inactivated, some where “Mom’s copy” is inactive ØTherefore, females are a mosaic of cells ØPreserved in mitosis ØIn ovaries, Barr body chromosome is reactivated for meiosis and oogenesis

IV. SEX-LINKED INHERITANCE, cont • X Inactivation Ø Calico coloration in female cats

V. PEDIGREE ANALYSIS

Recessive Trait Dominant Trait

VI. CHROMOSOMAL BASIS OF INHERITANCE • Chromosomal Theory of Inheritance o States genes occupy specific loci on chromosomes o During meiosis, chromosomes undergo segregation & independent assortment • Linked Genes o During Thomas Morgan’s work with Drosophila, he recognized Ø Two genes located on same chromosome were linked; that is, inherited together Ø However, offspring phenotypes showed this wasn’t always true

VI. CHROMOSOMAL BASIS OF INHERITANCE, cont • Linked Genes, cont o In fruit flies, normal wild-type phenotype is gray body, normal wings – both genes are located on same chromosome Ø G = wild-type (gray) body; g = black body Ø W = wild-type wings; w = mutant wings o True-breeding wild type flies X true-breeding mutants o F 1 showed all o F 1 X test cross Ø Counted 2300 offspring Ø Should have counted

VI. CHROMOSOMAL BASIS OF INHERITANCE, cont 965 GWgw 944 gwgw 206 Gwgw Morgan realized variation in probabilities due to 185 g. Wgw

VI. CHROMOSOMAL BASIS OF INHERITANCE, cont • Linkage Maps o In crossing over, the further apart two genes are, the higher the probability that a crossover will occur between them and therefore, the higher the recombination frequency.

VI. CHROMOSOMAL BASIS OF INHERITANCE, cont • Linkage Maps, cont o Recombination Frequency = # recombinants__ X 100 total # offspring o One map unit = 1% recombination frequency • Morgan’s Data … o F 1 X test cross Ø GW/gw X gw/gw Ø Counted 2300 offspring Ø Should have counted Ø 965 GWgw 944 gwgw 206 Gwgw 185 g. Wgw

VI. CHROMOSOMAL BASIS OF INHERITANCE, cont • In a genetics experiment with fruit flies, P generation true-breeding flies are crossed. The female parent is brown and wingless while the male parent is black with normal wings. All of the F 1 flies are brown with normal wings. • In the next cross, F 1 females are test-crossed with black, wingless males. The following offspring are counted: Ø 85 brown winged flies Ø 728 black winged flies Ø 712 brown wingless flies Ø 75 black wingless flies • How many map units apart are the two genes?

VI. CHROMOSOMAL BASIS OF INHERITANCE, cont • The genes for vestigial wings, black body color, and cinnabar eyes are linked genes. • In controlled crosses. . . Ø The gene for vestigial (vg) wings and body color (b) have a 17% crossover rate. Ø The gene for eye color (cn) and body color (b) have a 9% crossover rate. Ø The gene for eye color (cn) and vestigial wings (vg) have a 9 ½% crossover rate. • Draw the chromosome.

VII. MUTATIONS • Change in the nucleotide sequence of DNA • May be spontaneous mistakes that occur during replication, repair, or recombination • May be caused by mutagens; for example, x-rays, UV light, carcinogens • Classification v Gene v Chromosomal

VIII. INHERITED GENETIC DISORDERS • Most often due to gene mutations • Classified according to type of chromosome on which affected gene is located • Autosomal Disorders o Affects 1 st 22 pair of chromosomes • Sex-linked Disorder o Affects last pair of chromosomes (X) • Grouped according to path of inheritance o Recessive o Co-Dominant o Dominant

VIII. INHERITED GENETIC DISORDERS Autosomal Recessive Disorders • Albinism • Cystic Fibrosis • PKU • Tay-Sachs

VIII. INHERITED GENETIC DISORDERS, cont o Autosomal Co-Dominant Disorders Ø Sickle Cell Anemia

VIII. INHERITED GENETIC DISORDERS, cont o Autosomal Dominant Disorders Ø Huntington’s Disease

VIII. INHERITED GENETIC DISORDERS, cont • Autosomal Dominant Disorders Ø Marfan Syndrome

VIII. INHERITED GENETIC DISORDERS, cont • Autosomal Dominant Disorders Ø Achondroplasia

VIII. INHERITED GENETIC DISORDERS, cont • Hypercholesteremia

VIII. INHERITED GENETIC DISORDERS, cont • Sex-Linked Disorders o Vast majority are recessive o Affect mostly males o Hemophilia

VIII. INHERITED GENETIC DISORDERS, cont o Colorblindness o Duchenne Muscular Dystrophy

IX. CHROMOSOMAL MUTATIONS • Due to abnormal chromosome structure or abnormal number of chromosomes • Abnormal Chromosome Structure o Often due to mistakes in

IX. CHROMOSOMAL MUTATIONS, cont • Abnormal Number of Chromosomes o Most commonly due to nondisjunction o Results in aneuploid gametes o Detected with __karyotype_______

IX. CHROMOSOMAL MUTATIONS, cont • Chromosomal Number Disorders o Examples Ø Down Syndrome

X. CHROMOSOMAL DISORDERS, cont Turner Syndrome

X. CHROMOSOMAL DISORDERS, cont • Klinefelter Syndrome

XI. TESTING FOR INHERITED GENETIC DISORDERS • Identification of Carriers/Genetic Counseling o Tests are available for Tay-Sachs, sickle cell, cystic fibrosis, Huntington’s, PKU, & many others • Fetal Testing o Gene Testing v. Amniocentesis Ø Performed between 14 th-16 th weeks of pregnancy Ø Cells collected, tested

XI. GENETIC TESTING, cont v Chorionic Villus Sampling (CVS) Ø Narrow tube inserted through mother’s vagina, cervix Ø Small tissue sample suctioned from placenta (organ that transmits nutrients, removes wastes from fetus) Ø Testing may be done earlier in pregnancy but not suitable for all types of testing o Karyotype

XI. GENETIC TESTING, cont • Newborn Screening o PKU • PGD o Pre-Implantation Genetic Diagnosis