Probability & Genetic Crosses

Probability If you roll one die, what’s the probability of rolling a 6? If you pull one card from the deck, what’s the probability of getting the ace of hearts? If you flip a coin, what’s the probability of landing on heads? 1/6 (1 out of 6) 1/52 (1 out of 52) 1/2 (1 out of 2)

A Genetic Cross Gregor Mendel (1865) discovered that probability could be used to predict the results of a genetic cross (mating) If you know parent genotypes, you can predict offspring genotypes

A Genetic Cross If you know parent genotypes, you can predict offspring genotypes Example:  Brown hair (B) is dominant to blond hair (b).  What are the possible genotypes for their offspring? bb Bb ♀ Bb x ♂ bb Bb, Bb, bb, bb

A Genetic Cross If you know parent genotypes, you can predict offspring genotypes Example:  Brown hair (B) is dominant to blond hair (b).  What is the probability that the Incredibles’ next child will have brown hair? bb Bb ♀ Bb x ♂ bb Bb, Bb, bb, bb 2/4 (50%) chance of brown hair

Practice In pea plants, smooth peas (S) are dominant to wrinkled peas (s). A homozygous smooth pea is crossed with a wrinkled pea. ♀ _____ x ♂ _____

Practice In pea plants, smooth peas (S) are dominant to wrinkled peas (s). A homozygous smooth pea is crossed with a wrinkled pea. Determine the possible offspring genotypes & phenotypes. ♀ SS x ♂ ss Ss, Ss, Ss, Ss 100% chance of smooth peas

Practice In guinea pigs, dark fur (D) is dominant to light fur (d). Cross a heterozygous dark guinea pig with a light guinea pig. ♀ _____ x ♂ _____

Practice In guinea pigs, dark fur (D) is dominant to light fur (d). Cross a heterozygous dark guinea pig with a light guinea pig. Determine the possible offspring genotypes & phenotypes. ♀ Dd x ♂ dd Dd, Dd, dd, dd 50% chance of dark fur 50% chance of light fur

A Genetic Cross If you know offspring genotypes, you can predict parent genotypes Example:  Brown eyes (B) are dominant to blue eyes (b).  If Dash’s genotype is bb, what are his parents’ genotypes? bb ?? Bb

Practice In guinea pigs, dark fur (D) is dominant to light fur (d). Two dark-fur guinea pigs produce a light-fur offspring. What is the genotype of the offspring? dd

Practice In guinea pigs, dark fur (D) is dominant to light fur (d). Two dark-fur guinea pigs produce a light-fur offspring. What must the parent genotypes be? dd ?? Dd

Punnett Squares Created by Reginald Punnett (1900) to make predicting the outcome of genetic crosses easier show all the possible outcomes of a genetic cross show the probability of each outcome 25%

Punnett Squares show all the possible outcomes of a genetic cross show the probability of each outcome Example:  In dogs, short hair (S) is dominant to long hair (s).  Determine the possible offspring phenotypes. 25% ♀ Ss ♂ SS SsSs S 100% chance of short hair S SsSs S SsSs

Why Use a Punnett Square? Most scientists study MANY traits at one time It gets VERY confusing without a Punnett Square!

Practice In humans, straight toes (S) is dominant to curled toes (s). Determine the possible results for a cross between a recessive male & heterozygous female. 25% ♀ Ss ♂ ss 50% chance of straight toes 50% chance of curled toes SsSs s s SsSsSsSs

Practice In pea plants, yellow color (Y) is dominant to green (y). Determine the possible results for a cross of 2 heterozygous plants. 25% ♀ Ss ♂ Ss S s SsSs S SsSs SsSss 75% chance of yellow peas 25% chance of green peas

Special Cases Incomplete dominance  Neither allele is dominant  Heterozygous = in BETWEEN phenotypes Example: Carnations C R C R = CWCW =CWCW = CRCW =CRCW = ?

Special Cases Codominance  Neither allele is dominant  Heterozygous = BOTH phenotypes Example: Cows HWHW =HWHW = HBHB =HBHB = HWHB =HWHB = ?