#50 Using a Punnett Square

Vocabulary Heredity- The passing on of traits from parent to offspring Genetics- The study of heredity Gene- Units of heredity; segments of DNA on chromosomes Dominant- A trait that is shown when at least 1 dominant allele is present , represented by a capital letter (ex. B)“stronger trait” Recessive- A trait that is only shown if 2 recessive alleles are inherited , represented by a lower case letter (ex. b) “weaker trait” Allele- different forms of the same gene; represented by a letter (“B” or “b”) Genotype- Gene makeup of an organism (RR, Rr, or rr) Phenotype- Physical appearance (brown hair, green eyes) Homozygous- genotypes with the same alleles; either 2 dominant or 2 recessive Heterozygous- genotypes with different alleles; one dominant and one recessive Punnett Square- a tool to predict the probability of inheriting traits

What is a PUNNETT SQUARE? A tool to predict the probability of certain traits in offspring that shows the different ways the alleles of each genotype can combine A way to show possible phenotypes & genotypes

What is a PUNNETT SQUARE? Letters stand for dominant and recessive alleles An uppercase letter stands for a dominant allele Lowercase letters stand for recessive alleles

Using a PUNNETT SQUARE To set up a Punnett square, draw a large square, and then divide it into 4 equal sections (also squares). It should look something like this:

Using a PUNNETT SQUARE Now you need two parents to mate, ones with a known genotype For example, a red flower (genotype Rr) and a white flower (genotype rr). Rr x rr

Using a PUNNETT SQUARE Place one of the parents on top, and one on the left. You should get a something similar to this:

Using a PUNNETT SQUARE Finally, take each letter in each column and combine it with each letter from each row in the corresponding square. You should now have a picture close to this:

Using a PUNNETT SQUARE The two-letter combinations are the possible genotypes of offspring They are: Rr, Rr, rr, and rr genotypes From this it is possible to determine the probability (chance) that a flower will have a red phenotype (2/4 or 50%) or a white phenotype (2/4 or 50%)

1. Cross a homozygous dominant tall plant with a homozygous recessive short plant Recessive trait = short What letters would you use to represent this cross? Dominant trait = tall T = tall t = short

What is the genotype of a homozygous dominant tall plant? TT What is the genotype of a homozygous recessive short plant? tt TT x tt Genotype % TT % Tt % tt t t 100 Tt Tt T T Phenotype % tall % short 100 Tt Tt

#2. Cross a heterozygous brown haired rabbit with a homozygous recessive white haired rabbit. Dominant trait = brown hair Recessive trait = white hair What letters would you use to represent this cross? B = brown b = white

What is the genotype of a heterozygous brown haired rabbit? Bb What is the genotype of a homozygous recessive white haired rabbit? bb Bb x bb Genotype % BB % Bb % bb b b 50 Bb Bb B b 50 Phenotype % brown % white 50 bb bb 50

3. Cross a homozygous dominant long haired guinea pig with a heterozygous long haired guinea pig. Dominant trait = long hair Recessive trait = short hair What letters would you use to represent this cross? L = long l = short

What is the genotype of a homozygous dominant long haired guinea pig? LL What is the genotype of a heterozygous long haired guinea pig? Ll LL x Ll Genotype % LL % Ll % ll 50 L l 50 LL Ll L Phenotype % long % short 100 LL Ll

4. Cross a heterozygous wrinkly elephant with another heterozygous wrinkly elephant. Dominant trait = wrinkly skin Recessive trait = smooth skin What letters would you use to represent this cross? W = wrinkly w = smooth

What is the genotype of a heterozygous wrinkly-skinned elephant? Ww Ww x Ww Genotype % WW % Ww % ww 25 W w 50 W w WW Ww 25 Phenotype % wrinkly % smooth 75 Ww ww 25

A plant that is homozygous dominant for round seeds is crossed with a heterozygous plant. What is the probability that the offspring will have wrinkled seeds? R = round r = wrinkled RR x Rr Genotype % RR % Rr % rr 50 R r 50 R RR Rr Phenotype % round % wrinkled 100 RR Rr