GENETICS - CLUTCH CH.2 MENDEL'S LAWS OF INHERITANCE.

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2 CONCEPT: MENDELS EXPERIMENTS AND LAWS Mendel s Experiments Gregor Mendel was an Austrian monk who studied Genetics using pea plants Mendel used pure lines meaning that all offspring produced by pure line mating will be for that trait - Ex: Yellow-seeded pure line mating will produce yellow-seeded offspring Mendel labeled each in a specific way - Parental (P) Generation: Is the first mating that occurs - First Filial (F1) Generation: is the offspring produced from parental mating - These often undergo self-mating where one plant s pollen is used to fertilize itself - Can also undergo cross-fertilization where one plant s pollen is used to fertilize another plant - Second Filial (F2) Generation: is the offspring produces from F1 mating EXAMPLE: One of Mendel s Crosses Parental X F1 F1 were selfed meaning self-mating F = 8023 total F2 plants ¾ ¼ 3:1 ratio Page 2

3 Each F2 plant was selfed 1. F3 from yellow F2 ¾ ¼ 2. F3 from green F2 100% Then, he did a different cross. He mated a F1 yellow with a green X ½ ½ At the end of these crosses he knew 1. Yellow seeded plants always produced at least some yellow seeded offspring 2. Selfed, green seeded plants only produced other green seeded offspring 3. The green seeded plant trait could skin generations Page 3

4 Mendel s Laws By studying pea plants, Mendel came up with certain properties and that govern inheritance The properties include: - There is a heredity factor (gene) that is necessary for producing a certain trait - This gene comes in two forms (alleles) - One form (allele) is dominant to the other Mendel s Laws include: 1. Law of segregation: Alleles separate (during meiosis) to form gametes. - Each gamete contains a allele for each trait 2. Law of Dominance: Some alleles are dominant, and others are recessive 3. Law of independent Assortment: Genes for different traits segregate into gametes independently - Genes are randomly, and independently, put into gametes EXAMPLE: A cross of white (W) and red (R) flowers Page 4

5 PRACTICE: 1. Which of the following Mendel s postulates states that alleles separate in the formation of gametes? a. Law of segregation b. Law of dominance c. Law of independent assortment d. Law of dividing cells 2. True or False: Breeding two pure-lines of yellow-seeded flowers will always produce yellow-seeded offspring a. True b. False Page 5

6 3. What is the official genetics term for the second generation of offspring? a. Parental generation b. F1 generation c. F2 generation d. Grandchildren Page 6

7 CONCEPT: DIPLOID AND HAPLOID GENETICS Diploid Genetics Understanding allele combinations is extremely important in understanding genetics There are many important to remember: - Alleles are variants for a particular trait, in diploid organisms there are two alleles per gene - Alleles can be dominant or recessive; The dominant trait is always seen when it is present - Homozygous means that there are two of the same alleles; heterozygous means two different alleles - Each gene sits at a specific chromosomal locus EXAMPLE: Alleles Identical Alleles at 1 gene locus (Homozygous) Heterozygous Genes lie on - In Diploid cells, there are two chromosome copies - Each chromosome contains one allele - During meiosis these chromosomes are replicated once, but divided into daughter cells twice - Creates four haploid gametes (sex cells) Page 7

8 EXAMPLE: Chromosomes and Meiosis Homologous Pair A a 1 cell (2n) Sister Chromatid Replication A A a a 1 cell (2n) Dyad Tetrad Division #1 A A a a 1 cell (n) 1 cell (n) Division #2 A A a a 1 cell (n) 1 cell (n) 1 cell (n) 1 cell (n) Page 8

9 Chromosomes have a distinct - A centromere is a condensed region of the chromosome - It can be metacentric (in center), submetacentric (off-center), acrocentric (at one end), - The p arm is the shorter arm and the q arm is the longer arm - Determined by the length between centromere and end of chromosome EXAMPLE: Metacentric acrocentric p arm Centromere q arm submetacentric Page 9

10 Haploid Genetics In haploid cells, there is only one per gene Wild type and mutant alleles have different symbols - WT allele looks like a + ; Mutant allele looks like a - When opposite mating types fuse it creates a diploid combination (a + /a) called a meiocyte - These can be replicated and divided into haploid cells containing either a + or a EXAMPLE: Haploid cell creation a + a Fuse a + /a Replicate a + /a + /a/a Divide a + a + a a Page 10

11 PRACTICE: 1. Which of the following describes an acrocentric chromosome? a. The p arm is longer than the q arm b. The centromere is located at the center of the chromosome c. The centromere is located at the end of the chromosome d. The p arm and q arm are the same length 2. In diploid organisms there are chromosomal copies. In haploid organisms there is chromosomal copy. a. One, two b. Two, one c. Two, four d. Four, two Page 11

12 3. After a diploid cell undergoes meiosis, it divides to produce a. Two diploid cells b. Two haploid cells c. Four diploid cells d. Four haploid cells Page 12

13 CONCEPT: MONOHYBRID CROSS A monohybrid cross is a mating between two organisms with different alleles at a single gene Remember: The alleles can be presented in different ways - Dominant, recessive (A=dominant, a=recessive) - Wild Type, Mutant (+ = WT, a = mutant) 1. Two heterozygous purple plants 2. WT winged fly with mutant short-winged fly Genotypes Mother: Father: Phenotypes Mother: Father: Genotypes Mother: Father: Phenotypes Mother: Father: Genotypes Genotypes Phenotypes Phenotypes Page 13

14 PRACTICE 1. A black and white rabbit were mated. All F1 offspring were black, and the F2 offspring is made up of approximately ¾ black and ¼ white rabbits. a. Draw out two Punnet squares detailing both matings. b. Supposed two white F2 offspring were mated. What would be the phenotype and genotype of the F3 offspring? (a) White, aa (b) White, Aa (c) Black, Aa (d) Black AA Page 14

15 2) Green scales (G) in a particular species of fish is dominant over blue scales (g). The following crosses are carried out, producing the progeny shown. Write out all possible genotypes of the parents in each cross. Parents Progeny Genotypes of Parents a) Green x Green 4 green, 2 blue b) Green x Green 8 green c) Green x Blue 12 green d) Green x Blue 3 green, 1 blue e) Blue x Blue 2 Blue 3) Which of the following offspring ratios is expected from a Mendelian heterozygous cross examining one gene? a) 2:2 b) 3:1 c) 9:3:3:1 d) 4:2:1 Page 15

16 4) Human albinism is a simple recessive trait. Determine the genotypes of the parents for each offspring combination i. A wild-type male and albino female have 6 wild-type children a. AA x aa b. Aa x Aa c. aa x aa d. AA X AA e. AA x Aa ii. A wild-type male and albino female have 8 children, 4 wild-type, and four albino a. AA x aa b. Aa x Aa c. Aa x aa d. AA X AA e. AA x Aa Page 16

17 CONCEPT: DIHYBRID CROSS Punnet Square A dihybrid cross is a mating occurring between organisms containing two different traits Typically written like BbSs (heterozygous) Done for genes that independently assort - Inheriting one trait will not affect the inheritance of the other trait (Ex. color and shape) Two methods of doing a dihybrid cross 1. Punnet Square 2. Branch Diagram 1. Punnet Square Starting Genotypes Mother: Yy Rr Father: Yy Rr Starting Phenotypes Mother: Yellow, round Father: Yellow, Round 1. What is the probability of having a yellow round offspring? 2. What is the probability of having a yellow wrinkled offspring? 3. What is the probability of having a green round offspring? 4. What is the probability of having a green wrinkled offspring? The common dihybrid ratio is 9:3:3:1 Page 17

18 Branching Diagram 2. Branching Diagram Branching diagram uses math to calculate the the probability of certain genotypes? Starting Genotypes Mother: Yy Rr Father: Yy Rr Starting Phenotypes Mother: Yellow, round Father: Yellow, Round Steps 1. What is the probability of the offspring being yellow? Or green? 2. What is the probability of the offspring being round? Or wrinkled? Yy, Yellow Rr round rr wrinkled Yellow Round Yellow Wrinkled yy, Green Rr round rr wrinkled Green Round Green Wrinkled Page 18

19 PRACTICE 1. Assume you have mated a homozygous dominant purple, square plant with a homozygous recessive pink, spherical plant. What is the proportion of purple and spherical plants that would be produced in the F2 generation? Page 19

20 2. Write out all of the following gametes that can be produced from individuals with the following genotypes. a. AaBB b. AaBb c. AaBbCc d. AaBbcc 3. Two organisms with the genotypes Aa bb Cc Dd Ee and Aa Bb Cc dd Ee were crossed. Use the branch method to determine the proportion of the following genotypes in the offspring. I. aa bb cc dd ee a. 1/256 b. 1/64 c. 1/16 d. 1/4 Page 20

21 II. Aa bb Cc dd ee a. 1/256 b. 1/64 c. 1/16 d. 1/4 III. AA BB CC Dd ee a. 1/256 b. 1/64 c. 1/16 d. 0 Page 21

22 3. In melons, spots (S) are dominant to no spots (s) and bitterness (B) is dominant to sweet (b). Answer the following questions that arise from a crossing of a homozygous dominant plant with a homozygous recessive plant. Assume Mendelian inheritance. I. What is the F2 phenotypic ratio if the F1 generation is intercrossed? a. 12:3:1 b. 4:3:2:1 c. 9:3:3:1 d. 3:1 Page 22

23 CONCEPT: SEX-LINKED GENES Humans have two chromosomes, X and Y The Y chromosome has certain characteristics - The Y chromosome contains only a few dozen genes - SRY gene determines maleness - It is hemizygous because there is only one Y chromosome The X chromosome, unlike the Y, contains hundreds of genes for multiple functions - Both the X and Y have pseudoautosomal regions 1 and 2, which help pair the X and Y together - During meiosis, the X and Y act as a pair, so that they can segregate equally into sperm - Nondisjunction occurs when chromosomes fail to separate properly - XXX, XXY, XO, OY EXAMPLE: Sex-linkage is when genes located on the sex chromosomes show certain patterns - X-linkage is when there are mutant alleles on the X chromosome - Y-linkage is when there are mutant alleles on the Y chromosome - Sex-limited inheritance is when expression of a phenotype is absolutely limited to one sex - Example: Different coloration or size in males/females - Sex-influenced inheritance is when the sex of an individual influences the expression of a phenotype - Gene expression is dependent on male/female hormones - Example: Pattern Baldness Page 23

24 EXAMPLE: X-Linkage and eye colors in Drosophila P X Red-eyed female w + /w + White-eyed male w/y F1 Cross F1 males and females Red-eyed females/males w + /w or w + /y F2 Red-eyed males/females 3/4 White-eyed males 1/4 Page 24

25 EXAMPLE: X-linkage and eye colors in Drosophila - reciprocal cross P X Red-eyed male w + /y White-eyed female w/w F1 Red-eyed Females w + /w 1/2 White-eyed males w/y 1/2 F2 Red-eyed females/males w + /w or w + /y 1/2 White-eyed females/males w/w or w/y 1/2 Page 25

26 PRACTICE 1. Which of the following sex chromosome pairs is caused from nondisjunction? a. XX b. XY c. ZZ d. XXY 2. Which of the following is an example of sex-limited inheritance? a. Pattern baldness in humans b. Male doves are white, while female doves are brown c. Klinefelter s disease d. Men tend to be taller than women Page 26

27 CONCEPT: PROBABILITY AND GENETICS To predict the genotypes and phenotypes of offspring, geneticists use probability Product Law multiply the probability of independent events occurring together - Ex: Tossing a penny and a nickel each has a ½ chance of being heads - Probability of both being heads will be ½ x ½ = ¼ or 25% - Use this when two independent events are occurring together Sum Law add the probability of independent occurring together - Ex: Tossing a penny and a nickel each has ½ chance of being heads - Probability of one being heads and other being tails will be ¼ + ¼ = ½ or 50% - Use this when the events could occur in more than one way Binominal Theorem: Used when there are alternative ways to achieve a combination of events 1. What is the probability that in family with four children, two will be male and two will be female? Option 1 - (a + b) n a = male probability = ½ and b = female probability = ½ - (a + b) 4 = a 4 + 4a 3 b + 6a 2 b 2 + 4ab 3 + b 4 - Each of these terms represents a different outcome - a 4 = probability of having four males - 6a 2 b 2 = probability of having two males, two females 6(1/2) 2 (1/2) 2 = 3/8 Page 27

28 Option 2 - N = s + t ; n=total number of events, S = # of times a occurs t = # of time b occurs - 4 = p = 4! 2! 2! (1/2) 2 (1/2) 2 p = 3/8 PRACTICE 1. Use the product law to calculate the probability that mating two organisms with the genotype of AaBbCcDd will produce offspring with the genotype of AA bb Cc Dd? a. 1/4 b. 1/16 c. 1/64 d. 1/128 Page 28

29 2. In a family of five children what is the probability that I. Three are males and two are females a. 0.31, 31% b. 0.5, 50% c. 0.25, 25% d. 0.10, 10% II. All are females a , 3.1% b. 0.31, 31% c. 0.25, 25% d. 0.10, 10% III. Two are males and three are females a. 0.31, 31% b. 0.5, 50% c. 0.25, 25% d. 0.10, 10% Page 29

30 3. In a family of six children, where both parents are heterozygous for albinism, what is the probability that four are normal and two are albinos? a. 0.50, 50% b. 0.25, 25% c. 0.30, 30% d. 0.10, 10% Page 30

31 CONCEPT: PEDIGREES A pedigree is a map of human matings A propositus is the family member who first comes to the attention of a geneticist Pedigrees use many symbols Male Female Affected Male Affected Female Mating Heterozygotes for autosomal recessive Parents and Chidlren Death Propositus Twins In Genetics, you ll be given a pedigree and asked to identify the inheritance pattern Page 31

32 Autosomal Inheritance 1. Autosomal Recessive Disorders - Affected individuals appear in offspring of unaffected parents - Affected offspring include both males and females - There are only a few affected offspring EXAMPLE: 2. Autosomal Dominant Disorders - Phenotype appears in every generation of pedigree - Affected parents pass the phenotype to their children - Are rare, but normal allele is recessive EXAMPLE: Page 32

33 3. Autosomal Polymorphisms - Polymorphism existence of two or more common phenotype of a trait - Examples include: widow s peak, tasting PTC, attached/free earlobes - Inherited in standard Mendelian manner EXAMPLE: Nontasters (tt) Tasters (Tt or TT) Sex-linked inheritance 4. X-Linked Recessive Disorders - More males than females are affected - An affected male parent will not have affected offspring, but all daughters will be carriers - Sons of affected males will not pass it to their offspring - Ex: red-green colorblindness EXAMPLE: Page 33

34 5. X-linked Dominant Disorders - Affected males pass condition to all daughters, but no sons - Affected heterozygous females who mate with unaffected males pass condition to ½ of all offspring EXAMPLE: 6. Y-linked Disorders - Only males inherit it - Very rare EXAMPLE: Page 34

35 Pedigree Flowchart Do all affected individuals have an affected parent? Yes No Do all the affected males have an affected mother Does the trait affect mostly males? Yes No Yes No Does an affected father produce daughters who are all affected? Yes No Autosomal Dominant Are all the sons of an affected father also affected? No Autosomal Recessive Does an affected father produce sons who are all not affected? Y-Linked X-Linked Recessive No Yes X-Linked Dominant Page 35

36 PRACTICE: 1. This pedigree exhibits which of the following inheritance patterns? a. Autosomal Dominant b. Autosomal Recessive c. X-linked Dominant d. Autosomal Polymorphism 2. This pedigree exhibits which of the following inheritance patterns? a. Y-Linked b. Autosomal Recessive c. X-linked Dominant d. Autosomal Polymorphism Page 36

37 3. This pedigree exhibits which of the following inheritance patterns? a. Autosomal Dominant b. X-linked Recessive Recessive c. X-linked Dominant d. Autosomal Polymorphism Page 37