# Revisiting Dihybrid Punnett Squares What exactly is a dihybrid cross? –Predicting the outcomes from crossing 2 traits. Why do we use them? –Instead of.

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Revisiting Dihybrid Punnett Squares What exactly is a dihybrid cross? –Predicting the outcomes from crossing 2 traits. Why do we use them? –Instead of doing to monohybrid crosses, you can combine them to see all the possible combinations from that particular mother and father.

Dihybrid Crosses Cont’d What do the tops and sides of a Punnett Square represent? –The gametes from the mother and father that are going to mix (fertilize) to make the possible offspring in the middle. Webbed feet (W) in ducks are dominant to un-webbed (w) feet. If a mother duck is heterozygous for webbed feet, and the father is homozygous for webbed feet, will any of their ducklings have un-webbed feet? Mother = Ww, Father = WW WW W w W W w Each gamete is haploid. The mother’s eggs can have either W or w; father’s sperm can have either W or W.

Dihybrid Crosses Cont’d Why do we have a Punnett Square with 4 boxes on each side instead of 2? –There will be 2 possible alleles that can be passed on for each trait. You will have 4 total options to include all the possibilities of combining the 2 traits. Short beaks (b) in ducks are recessive to long beaks (B) feet. If a father duck has a short beak, and the mother is a carrier for short beaks, will any of their ducklings have the same type of beak as their mother?? Mother = bb, Father = Bb The mother can pass on a B or a b; the father can pass on a b or a b towards the beak trait. Remember, the mother can pass on a W or a w allele for webbed feet and the father can pass on a W or a W… SO, all the possible gamete combinations are: Mother: WB,Wb, wB, wb Father: Wb, Wb, Wb, Wb

Putting it all together… Mother’s gamete choices: WB, Wb, wB, wb Father’s gamete choices: Wb, Wb, Wb, Wb By taking all the daughter cell possibilities, we’re covered no matter what gametes end up getting fertilized to restore diploidy. WB Wb wB wb Wb WWBb WWbb WwBb Wwbb

Codominance, Polygenic Inheritance and Blood Types Continuing our exploration of patterns of inheritance with…

Codominance Heterozygote expresses both alleles’ conditions –Ex. A black rooster bred with a white hen produces a black and white checkered chicken. Usually uses both capital letters (black= B, white=W, checkered = BW)

Codominance practice: cross a black rooster with a white hen B B WW Results: 100% checkered (BW) BW Key: B = black W = white BW = checkered

Now let’s cross a checkered hen with a checkered rooster. B W WB Results: 25% Black 50% Checkered 25% White (1:2:1) BBBW WW

Polygenic Inheritance Trait controlled by 2 or more genes May be on the same or different chromosomes Shows a range, intermediate is most common phenotype Upper and lower case letters used

Skin color: A polygenic trait Determining # of Genes Involved in Skin Color Observed distribution of skin color Expected distribution- 1 gene Expected distribution- 4 genes Expected distribution- 3 genes Range of skin color Light Right Number of individuals

And the answer is… Skin color is actually due to 5 genes Genotypes darkest to lightest: –AABBCCDDEE would be darkest skinned –AaBbCcDdEe would be medium skinned –aabbccddee would be lightest skinned

Influence of External Environment In arctic foxes temperature has an effect on the expression of coat color. In winter, fur is white; in summer, fur is brown

Leaves can have different sizes, thicknesses, and shapes depending on the amount of light they receive. Influence of External Environment

Influence of Internal Environment The internal environments of males and females are different because of hormones and structural differences. An organism’s age can also affect gene function. –Ex. Adult male lion’s manes

Peacock (male) Peahen (female) Coloration may also be caused by differences in hormones between sexes

Blood type quick facts Red blood cells are called erythrocytes Proteins on their surfaces are called antigens, controlled by genes Antigens make antibodies to foreign substances, which includes RBCs with different antigens on their surface 4 phenotypes: A, B, AB, O 3 alleles: I A, I B, i

The l A allele is dominant to i, so inheriting either the l A i alleles or the l A l A alleles from both parents will give you type A blood. Phenotype A Surface molecule A is produced. Surface molecule A

The l B allele is also dominant to i. Phenotype B To have type B blood, you must inherit the l B allele from one parent and either another l B allele or the i allele from the other. Surface molecule B is produced. Surface molecule B

The l A and l B alleles are codominant. Phenotype AB If you inherit the l A allele from one parent and the l B allele from the other, your red blood cells will produce both surface molecules and you will have type AB blood. Surface molecule B Surface molecule A

Phenotype O No antigens produced

Check your chart! Blood Group AntigensAntibodiesCan receive from Can give to AABA or OA or AB BBAB or OB or AB ABA and BBothA, B, AB, O AB ONone OA, B, AB, O

Possible genotypes for each phenotype: A=I A I A or I A i B=I B I B or I B i AB =I A I B O=ii

Now let’s put your skills to the test… Can you save these patients?

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