genetics: incomplete dominance & codominance biology 12
TRANSCRIPT
Genetics: Incomplete Dominance & Codominance
Biology 12
Joke of the day:
Review of Mendel’s Principles
Genes are passed parents offspring; get one allele from each parent
During Meiosis, the alleles for a gene segregate from each other.
During Meiosis, genes independently assort with each other.
Review: Dominant/Recessive One allele is dominant over the other
(capable of masking the recessive allele)
PP = purple pp = white Pp = purple
Review Problem: In pea plants, purple flowers (P) are dominant over
white flowers (p) show the cross between two heterozygous plants.
P
p
P p
pp
Pp
Pp
PP- PP (1); Pp (2); pp (1)- ratio 1:2:1
- purple (3); white (1)- ratio 3:1
GENOTYPES:
PHENOTYPES:
Exceptions to Mendel’s principles
Some alleles are neither dominant nor recessive.
Many traits are controlled by more than one gene (polygenic traits)
Incomplete dominance: A situation in which neither allele is dominant. When both alleles are present a “new”
phenotype appears that is a blend of each allele. The heterozygous phenotype is
somewhere in between homozygous phenotype.
2 alleles produce 3 phenotypes
Incomplete Dominance:
CRCR = red CrCr = white CRCr = pink
Rr = pink
Problem: Incomplete Dominance
Show the cross between a pink and a white flower.
- Rr (2); rr (2)- ratio 1:1
- pink (2); white (2)- ratio 1:1
R r
r
r
rrRr
rrRr
GENOTYPES:
PHENOTYPES:
Codominance:
Neither allele are dominant; both are expressed.
A cross between organisms with two different phenotypes produces offspring with has both phenotypes of the parental traits shown.
Example: codominance In some chickens:
Black Chicken x White Speckled Chicken
Roan coat color in horses and cattle are codominant for the alleles R and r. Homozygous R individuals are red (or bay)Homozygous r individuals are whiteWhile heterozygous Rr individuals are roan (which means they have both white and red hair, giving them a lightened appearance.
Example:
What about the F2
generation? Phenotype?
1 red, 2 roan and 1 white 1:2:1
Genotype? 1Hr Hr, 2 HrHw and 1Hw Hw
Which type of dominance…
Codominance!
To do:
Page 145: questions 2, 3, 4, 5 Practise sheet: Codominance and
Incomplete dominance Colouring sheet