Topic 7A: Genetics Flashcards
What is monohybrid inheritance?
- Controlled by a single gene
What do monohybrid punnet squares show?
- Likelihood of inheritance of a characteristic
What things should you show when doing a punnet square?
- Parental phenotypes, genotypes, gametes
- Offspring genotypes, phenotypes
- Ratio of offspring phenotypes
What is the expected ratio of a homozygous dominant and homozygous recesssive?
- All dominant
What is the ratio for a heterozygote and a homozygous recessive?
- 1: 1
What is the ratio for 2 heterozygotes?
- 3 : 1 –> dominant : recessive
When is a test cross used and what does it show?
- Used to find the genotype of an individual displaying the dominant characteristic
- Test by crossing with homozygous recessive
- If some offspring show the recessive characteristic –> parent was heterozygous
Why is the actual ratio of offspring not the same as the expected?
- Fertilisation is random
How can results be made more reliable?
- Use a larger sample size - more representative of the population
What are codominant alleles?
- Neither is recessive
- Both are expressed in the phenotype if heterozygous
What are multiple alleles?
- More than 2 alleles present for the same gene
- Some dominant, some recessive
What is dihybrid inheritance?
- Inheritance of 2 different genes
What gametes would be used from RRYY?
- RY only
What are the gametes for RrYY?
- RY, rY
What are the gametes for rrYy?
- rY, ry
What are the gametes for RrYy?
- RY, Ry, rY, ry
What is the ratio for 2 dihybrid heterozygotes?
- 9:3:3:1
- 9 = both dominant
- each 3 = 1 dominant
- 1 = both recessive
What is linkage?
- Genes found on the same chromosome
What is autosomal linkage?
- Linked genes on a chromosome that is not a sex chromosome
How can genes be more closely linked?
- Closer together on chromosome
- Less likely to have crossing over occur
Is crossing over common?
- It is very rare so non linked gametes much less common
For AaBb where bith dominant and both recessives are linked what are the gametes?
- AB, ab
What is sex linkage?
- Genes linked on a sex chromosome
Why are sex linked recessive disorders more common in males?
- Males - only have 1 X chromosome –> allele present always expressed
- Females - 2 X chromosomes –> only expressed if homozygous recessive
What is epistasis?
- When the allele of one gene affects / masks the expression if the allele of another gene
Which stats test is used in genetics and why?
- Chi squared
- Used when looking for a difference in the number / frequency of data in categories
- Look for difference in observed and expected ratios of phenotypes
What is the equation?
X2 = ∑▒〖(O-E)〗^2/E
- O = observed frequency
- E = Expected frequency
What are the degrees of freedom?
- Number of categories - 1
What do the null hypotheses look like?
- There is no significant difference in the observed and expected numbers in the different categories (phenotypes)
What do you want the outcome to be?
- Accept null hypothesis - should be no significant difference
