In complete dominance, heter0zyg0us individuals carry both alleles -dominant and recessive- but only express the dominant phentoype. Cross 1: Nn x nn. Cross 2: NN x nn
<h3>What is complete dominance?</h3>When the dominant allele completely masks the recessive allele, we talk about complete dominance.
This is the case of individuals that are heter0zyg0us for a particular gene and express the dominant trait. The dominant allele is hiding the expression of the recessive allele.
<h3>What is a punnett square?</h3>The Punnett square is a representation that shows the different types of <u>gamete combinations </u>according to the alleles involved in a cross.
Punnett square shows the <u>probabilities</u> of getting offspring with different <u>genotypes</u> and their consequent<u> </u><u>phenotypes.</u>
So, according to the provided data,
- <u>Cross 1</u>: gray fur x black fur
F1) 3 gray fur: 2 black fur
- Cross 2: gray fur x black fur
F1) 0 gray fur: 5 black fur
By looking at the phenotype of progeny produced by family 2, and assuming complete dominance, we can infere that black is the dominant phenotype. Why is that? because when you crossed a gray animal with a black animal and the whole progeny is black.
So, assuming a diallelic gene coding for the fur color, we will name
- N to the dominant allele that codes for black
- n to the recessive allele that codes for gray.
Possible genotypes and phenotypes
- NN ⇒ Black fur ⇒ dominant phentoype
- Nn ⇒ Black fur ⇒ dominant phentoype
- nn ⇒ Grey fur ⇒ recessive phentoype
Here what you need to analyze is which phentoypes appear among the progeny and their frequencies to get to interprete the parents' genotype.
- If both parents are h0m0zyg0us dominant (NN x NN), the whole progeny expresses the dominant trait (100% NN)
- If one parent is h0m0zyg0us dominant and the other one heter0zyg0us (NN x Nn), the whole progeny expresses the dominant trait (50%NN and 50%Nn).
- If one parent is h0m0zyg0us dominant and the other one h0m0zyg0us recessive (NN x nn), the whole progeny expresses the dominant trait (100% Nn).
- If both parents are heter0zyg0us (Nn x Nn), the offspring will express both phenotypes in a ratio 3:1 (25% NN + 50% Nn and 25% nn).
- If one parent is h0m0zyg0us recessive and the other one heter0zyg0us (nn x Nn), the offspring will express both phenotypes in a ratio 1:1 (50% Nn and 50% nn).
- If both parents are h0m0zyg0us recessive (nn x nn), the whole progeny will express the recessive trait (100% nn).
Now let us analyze the crosses,
- <u>Cross 1</u>: gray fur x black fur
F1) 3 gray fur: 2 black fur
You crossed a black animal with a gray animal and the offspring expresses both phenotypes. So the black animal (dominant phenotype) must be heter0zyg0us for coat color.
Parents' phenotype) gray and black
Parentals) nn x Nn
Alleles in Gametes) n n N n
Punnett square) N n
n Nn nn
n Nn nn
F1) Genotype
1/2 = 50% of the progeny is expected to be heter0zyg0us, Nn
1/2 = 50% of the progeny is expected to be h0m0zyg0us recessive nn
Phenotype
1/2 = 50% of the progeny is expected to be black -Nn-
1/2 = 50% of the progeny is expected to be gray -nn-
- Cross 2: gray fur x black fur
F1) 0 gray fur: 5 black fur
You crossed a black animal with a gray animal and the offspring expresses only the dominant phenotypes. So the black animal must be h0m0zyg0us dominant for coat color.
Parents' phenotype) gray and black
Parentals) nn x NN
Alleles in Gametes) n n N N
Punnett square) N N
n Nn Nn
n Nn Nn
F1) Genotype
100% of the progeny is expected to be heter0zyg0us, Nn
Phenotype
100% of the progeny is expected to be black -Nn-
Remember that the Punnett square does not give you information about the number of individuals that can be produced. It just provide information about probabilities of getting individuals with certain genotype of phenotype and their frequencies.
You can learn more about complete dominance and punnett square at
#SPJ1