Genetics

1. Genetics THE STUDY OF HEREDITY LH – Winter 2011

2. Gregor Mendel • The scientific study of heredity is called GENETICS! • Augustinian Monk • Began working on pea plants in his monastery • Correctly believed that heritable factors (genes) retain their individuality from generation to generation • i.e. – marbles

3. Why Pea Plants? • Mendel chose to study garden peas, because: • They reproduce quickly & have a short life cycle • They have seven distinct & observable traits • They produce many offspring in one cross • Ease in manipulating pollination • Self-fertilization vs. Cross Pollination • Mendel produced seeds by cross pollinating • Note: Since pea plants can self-pollinate, they are called true-breeding. This means they will produce offspring identicalto themselves.

4. Pea Plant Characteristics

5. Mendel (con’d) • Mendel worked with his pea plants until he was sure that all were true-breeding varieties • He could finally begin his studies: What would happen if different characteristics were crossed? • Purple flower x White flower? • Hybrid the offspring of two different varieties (also called a monohybrid cross) • P generation (P1)  parental generation have offspring called the F1 generation • If F1 generations cross, their offspring are called the F2 generation

6. P  F1  F2

7. Mendel’s Experiments • After studying pea plants, Mendel concluded that: • Traits are passed from one generation to the next through genes. • Each trait is controlled by a different form of a gene called an allele • Some alleles are dominantto others called recessive traits • New question: Have the recessive alleles disappeared or are they still present in the parents?

8. Mendel’s Experiments Mendel crossed the first generation and saw that the recessive trait showed up in about 1 of 4 plants. Conclusion: Law of Segregation! Segregation of the alleles happens during the formation of gametes. Each gamete will carry one form of the allele.

10. Using Probability to Predict Offspring • Punnett square – a diagram that shows the gene combinations that mightresult from a genetic cross of two parents • Phenotype – a description of what an individual LOOKS like (tall, red) • Genotype – a description of the genetic make-up of an individual (TT, Rr)

11. Important Vocabulary • Dominant – allele that appears more frequently. It masks the recessive. • Represented by a capitol letter (R=red) • Recessive – allele that appears less frequently (b/c it is repressed when paired with a dominant allele) • Represented by a lower case letter (r=white) • AA – HOMOZYGOUS dominant • aa– homozygous recessive • Aa– HETEROZYGOUS one of each allele • Homozygous – two identical alleles for a trait • Heterozygous – two different alleles for a trait

12. Solving Punnett Squares LH Biology Winter 2011

13. Punnett squaresStep 1 STEP 1  Define the alleles If a homozygousround pea plant is crossed with a heterozygousround pea plant, what will their offspring look like? R = round r = wrinkled

14. Step 2 Define the parents If a homozygousround pea plant is crossed with a heterozygousround pea plant, what will their offspring look like? RR x Rr

15. Step 3Draw the Punnett square R R R r

16. Step 4 Cross the parents  find the probability of offspring R R R r

17. Step 5 Find the genotypeand phenotypeof the offspring Genotype: genetic make-up (letters) R R R Phenotype: physical characteristics r

18. Finished Product R=round R=wrinkled RR x Rr R R Genotype ratio: 2 RR: 2Rr R Phenotype ratio: 100% Round r

19. Dihybrid Crosses • Dihybrid Cross – a cross of parents differing in TWO characteristics • For example: homozygous round, yellow x homozygous wrinkled, green seeds • RRYY x rryy • Law of Independent Assortment – each pair of alleles for different traits segregate independently of other pairs of alleles during gamete formation • This explains genetic diversity among organisms

20. Setting up a dihybrid • #1- list all 4 alleles • For example: R=round, r=wrinkled, Y=yellow, y=green • #2 – Create the parental genotypes (4 letters each) • Example: RRYY (Round, yellow) x rryy (wrinkled, green) • #3 – Using the “foil” method, determine the sets of gametes (up to 4 possibilities) • Example: RRYY  RY RrYy  RY, Ry, rY, ry • #4 – Fill in the tops and sides of punnett square with gamete combinations • #5 - Genotype and Phenotype as usual

21. Dihybrid Example Problem #1 Round is dominant over wrinkled Yellow is dominant over green Two pea plants produce offspring. One is round and heterozygous for yellow seed color. The other is wrinkled and heterozygous for yellow seed color. Parental genotypes = RRYy x rrYy Possible gametes rY, ry RY, Ry

22. Dihybrid Example Problem #1 Set up the dihybrid cross RY Ry rY ry

23. Dihybrid Example Problem #1 Determine the genotype and phenotype! RY Ry Genotype: Phenotype: 1 RrYY: 2 RrYy : 1 Rryy 3 Round, yellow 1 Round, green rY ry

24. Dihybrid Example Problem #2 • Key: Black fur is dominant (B) to white fur (b) Long hair is dominant (L) to short hair (l) • Two guinea pigs mate. The dad is homozygous for black fur and long hair. The mom is also homozygous, but for white fur and short hair. • 1) Determine the possible gametes of each • 2) What is the only gamete possibility for their offspring?

25. Dihybrid Example Problem #2 Key: Black fur is dominant (B) to white fur (b) Long hair is dominant (L) to short hair (l) Two guinea pigs mate. The dad is homozygous for black fur and long hair. The mom is also homozygous, but for white fur and short hair. 1) Determine the possible gametes of each Dad  4 BL Mom  4 bl 2) What is the only gamete possibility for their offspring? 100% BbLl (Black, long-haired)

26. Incomplete Dominance Neither allele is dominant over the other Heterozygous condition shows a blending of genes Assign capital and lowercase letters for alleles

27. Example problem #1 • If a red four o’clock flower is crossed with a pink four o’clock flower what will their offspring look like? RR = red rr = white Rr = pink

28. Perform cross RR xRr Genotype ratio: Phenotype ratio:

29. Codominance Both alleles are equal Both alleles are expressed in the organism Use capital letters for both alleles

30. Codominance Example • Black feathers and white feathers in chickens is codominant. In the heterozygous condition the feathers are called “erminette” and appear blue. • BB = black • WW or B’B’ = White • BW or BB’ = blue

31. Codominance Problem #1 Cross a Black chicken with a blue rooster Parents = BB x BW (BBx BB’) Genotype ratio: Phenotype ratio:

32. Sex-linked Problems • Some traits are only carried on the X chromosome • Traits can be dominant or recessive • Sex-linked traits include: • Color blindness • Hemophilia • Duchene muscular dystrophy • Fragile X syndrome • Males sex chromosome (XY) • Females sex chromosome (XX) • Sex-linked traits are more common in males

33. It is good to be female! • Males are more likely to have these disorders since they only have 1 X chromosome (XY) • Females have two chances (XX), but they can be carriers and pass it on to their offspring • Ex: H = normal h = hemophilia XHXH = normal female XHY = normal male XHXh= carrier female XhY = hemophiliac male

34. Example Problem • A carrier female marries a normal male. What are the chances that they will have a hemophiliac child? • Key: • Parents • Genotype ratio: • Phenotype ratio:

35. Multiple Alleles Blood Type Problems There are 4 blood types: A, B, AB, O A and B are codominant O is recessive

36. Blood Type Key AA = homozygous Type A AO= heterozygous Type A BB= homozygous Type B BO= heterozygous Type B AB= codominant AB OO = Type O SIX total combination possibilities

37. Example Problem A type AB woman marries a type O man. What are the possible phenotypes of their offspring? AB x OO B A O O

38. Dihyrbid Example Problem Round is dominant over wrinkled Yellow is dominant over green Two homozygous pea plants produce offspring. One is round and yellow. The other is wrinkled and green. What will their F2 generation look like? RY rYRyry RY rY Ry ry