Genetics

1. Class # ____ Genetics Practice Problems

Genetics What is heredity/Genetics?

How do you look like your parents, or more like one parent than the other?

How is it possible to maintain such genetic continuity?

Martin SheenCharlie Sheen

Emilio Estevez Kirk Douglas Michael

Kirk

History - Gregor Mendel

Fig. 2.2

Gregor Mendel’s monastery garden.

Augustinian monk lived (1822-1884)

Documented a particulate mechanism of inheritance through his experiments with garden peas

Crossing Pea Plants

1

5

4

3

2

Removed stamensfrom purple flower

Transferred sperm-bearing pollen fromstamens of white flower to egg-bearing carpel of purple flower

Parentalgeneration(P)

Pollinated carpelmatured into pod

Carpel(female)

Stamens(male)

Planted seedsfrom pod

Examinedoffspring:all purpleflowers

Firstgenerationoffspring(F1)

Mendel’s Observations When Mendel crossed white and purple flowered pea plants all of the offspring were

purple

When Mendel crossed the F1 plants, many of the plants had purple flowers, but some had white flowers

A ratio of about three to one, purple to white flowers, in the F2 generation

P Generation

(true-breeding parents) Purple

flowersWhiteflowers

F1 Generation (hybrids)

All plants hadpurple flowers

F2 Generation

EXPERIMENT True-breeding purple-flowered pea plants andwhite-flowered pea plants were crossed (symbolized by ). Theresulting F1 hybrids were allowed to self-pollinate or were cross-pollinated with other F1 hybrids. Flower color was then observedin the F2 generation.

RESULTS Both purple-flowered plants and white-flowered plants appeared in the F2 generation. In Mendel’sexperiment, 705 plants had purple flowers, and 224 had whiteflowers, a ratio of about 3 purple : 1 white.

Phenotype vs Genotype

Figure 14.6

3

1 1

2

1

Phenotype

Purple

Purple

Purple

White

Genotype

PP(homozygous)

Pp(heterozygous)

Pp(heterozygous)

pp(homozygous)

Ratio 3:1 Ratio 1:2:1

Purple(Pp)

Purple(PP)

Pp p p

P

P

P

p

F1 generationAll purple

White(pp) Purple

(Pp)

Pp PpPPGametes

F2 generation¾ purple, ¼ white

GametesGametes

GametesPp

Pp PpPp

pp

Mendel’s Monohybrid Cross

Mendel’s Studied Discrete Traits

Mendel Observed The Same

Pattern In Characters

Genes can be

dominant or recessive. Dominant trait is one

which will be expressed if one of the parents has the gene for that trait.

Recessive trait is one that will be expressed only if both parents carry the trait.

But, most human traits are NOT simple!

Co-dominance

Incomplete dominance

Humans have 23 pairs of chromosomes for a total of 46. 22 are autosomal one set determine what sex we are: XX (female) XY (male)

Sex-linked traits

And most traits involve many genes!!

The Human Genome Project has confirmed 19,599 genes, at this point...

Relax, we won’t be getting into that level of complexity in 8th grade!

Simple “Mendelian” – Autosomal Dominant &

Recessive Ability to taste phenylthiocarbamide

(dominant)

Ability to smell (bitter almond-like) hydrogen cyanide

Albinism (recessive)

Brachydactyly (shortness of fingers and toes)

Immunity to poison ivy (dominant)

Hitchiker's thumb (recessive)

Wet (dominant) or dry (recessive) earwax

Genotype = Full hereditary information. It describes the genetic contribution

from both parents.

Example: Wet ear wax is (dominant) & dry is recessive

Written based on the DOMINANT trait using a letter. W Dominant wet ear wax gets a capital. W Recessive dry ear was gets a lower case letter. w

Homozygous = BOTH genes are the same WW homozygous dominant or ww homozygous recessive

Heterozygous = Genes are different Ww W for a dominant gene from one parent w for a recessive gene from the other parent

Do Problem Set #1

1. For each genotype, indicate whether it is heterozygous (HE)

or homozygous (HO). AA _HO

Bb _ HECc _ HEDd _HE

Eeff GG HH

IiJjkk Ll

Mm nn OO Pp

1. For each genotype, indicate whether it is heterozygous (HE)

or homozygous (HO). AA _HO

Bb _ HECc _ HEDd _HE

Ee _HEff _ HOGG _HOHH _HO

Ii __HEJj __HEkk _HOLl _HE

Mm HEnn HOOO HOPp HE

Phenotype = the physical expression of the resulting

genetic codeIn sneeches, stars are dominant and no stars are

recessive.

Genotypes could be:

SS or Ss for stars

What would the genotype for no stars have to be?

Do Problem Set # 2

Genetic Vocabulary

Phenotype – observable characteristic of an organism

Genotype – pair of alleles present in and individual

Homozygous – two alleles of trait are the same (YY or yy)

Heterozygous – two alleles of trait are different (Yy)

Capitalized traits = dominant phenotypes

Lowercase traits= recessive phenotypes

2. For each of the genotypes below, determine the phenotype.

Purple flowers are dominant to white flowersPP ______purple____________Pp ______purple_______________pp _____ white________________

Brown eyes are dominant to blue eyesBB Bbbb

Round seeds are dominant to wrinkledRRRr rr

Bobtails are recessive (long tails dominant)LLLl ll

2. For each of the genotypes below, determine the phenotype.

Purple flowers are dominant to white flowersPP ______purple____________Pp ______purple_______________pp _____ white________________

Brown eyes are dominant to blue eyesBB _______brown________________Bb _______brown________________bb _______blue________________

Round seeds are dominant to wrinkledRR ______round__________________Rr ______round________________rr _______wrinkled_______________

Bobtails are recessive (long tails dominant)LL________long________________Ll _________long__________________ll _________bobtails_____________

3. For each phenotype, list the genotypes. (Remember to use the letter

of the dominant trait)

Straight hair is dominant to curly. _____SS_____ straight_____Ss_____ straight_____ss_____ curly

Pointed heads are dominant to round heads.

3. For each phenotype, list the genotypes. (Remember to use the letter

of the dominant trait)

Straight hair is dominant to curly._____SS_____ straight_____Ss_____ straight_____ss_____ curly

Pointed heads are dominant to round heads.____PP____ pointed____Pp______ pointed____pp_______ round

Punnett Squares Tool for determining phenotypes from

genotypes.

Begin with the genotypes of the parents.

Meiosis splits the chromosome in half to produce 2 gametes: Dad 2 sperm Mom 2 eggs

Each has HALF!

Example: Wet & dry ear wax:

Parental P0 cross

F1 cross

P Ppp

P pPp

Determine the genotype and phenotype

Mendelian GeneticsPink (P) flowers are dominant over white flowers (p)

4. Set up the square for each of the crosses listed below. The trait being

studied is round seeds (dominant) and wrinkled seeds (recessive)

A. Rr x rr What percentage of the offspring will be round?

B. Rr x Rr  What percentage of the offspring will be round?

C. RR x Rr  What percentage of the offspring will be round?

4. Set up the square for each of the crosses listed below. The trait being

studied is round seeds (dominant) and wrinkled seeds (recessive)

A. Rr x rr What percentage of the offspring will be round?

4. Set up the square for each of the crosses listed below. The trait being

studied is round seeds (dominant) and wrinkled seeds (recessive)

B) Rr x Rr  What percentage of the offspring will be round?

R

r

R r

RRRr

Rrrr

4. Set up the square for each of the crosses listed below. The trait being

studied is round seeds (dominant) and wrinkled seeds (recessive)

C) RR x Rr  What percentage of the offspring will be round?

R r

RRRr

RRRr

R

R

Do problems 5 - 10

Show all your work in your Lab Journal.

Answer Key5. A TT (tall) plant is crossed with a tt (short plant). What

percentage of the offspring will be tall? 100%

6. A Tt plant is crossed with a Tt plant. What percentage of the offspring will be short? 25%

7. A heterozygous round seeded plant (Rr) is crossed with a homozygous round seeded plant (RR). What percentage of the offspring will be homozygous (RR)? 1/2 or 50%

8. A homozygous round seeded plant is crossed with a homozygous wrinkled seeded plant. What are the genotypes of the parents? RR x rr What percentage of the offspring will also be homozygous? 0 %

9. In pea plants purple flowers are dominant to white flowers. If two white flowered plants are cross, what percentage of their offspring will be white flowered? 100% white

10. A white flowered plant is crossed with a plant that is heterozygous for the trait. What percentage of the offspring will have purple flowers? pp x Pp results in 50% purple