Inheritance

Question 1

Explain what is meant by the terms Genotype and Phenotype

Answer 1

Genotype- genetic constitution of an organism
Phenotype-the expression of this genetic constitution and its interaction with the environment

Question 2

Define the term allele

Answer 2

Different versions of a gene

Question 3

Homozygous and Heterozygous

Answer 3

Homo- both alleles at a specific locus on each homologous chromosome are the same

Hetero-both alleles at a specific locus on each homologous chromosome are different

Question 4

Alleles may be….

Answer 4

Dominant- in heterozygous this is expressed
Recessive-only expressed if homozygous recessive
Codominant-both alleles for the same gene in heterozygous organism contribute to phenotype

Question 5

Monohybrid crosses

Answer 5

Used to illustrate possible genotypes (and thus, phenotypes) of
offspring from two parents, and the probability of occurrence of each

Question 6

Dihybrid crosses

Answer 6

• Inheritance of two different characteristics controlled by two
  different genes.
  -An allele from one pair of chromosomes can enter a gamete
  with either allele from the other pair because of independent segregation.

Question 7

Codominance

Answer 7

Both allele in a heterozygous individual contribute to phenotype

Question 8

Crosses involving multiple alleles

Answer 8

Individuals only have 2 alleles of a gene but there may be more than 2 alleles in a population

Question 9

Crosses involving sex-linkage

Answer 9

A gene is ‘sex-linked’ if its locus is on a sex -chromosome
- Specific characteristic more likely to be inherited in either
male or female offspring
- Females (XX) have two copies of the gene / allele
- Only express recessive allele if homozygous recessive
- Males (XY) have 1 copy of the gene / allele
- Can’t be heterozygous; express recessive X-linked allele if only one copy present,(inherited from mother)

Question 10

Autosomal linkage

Answer 10

-Two genes carried on the same autosome
-Genes stay together during independent segregation
-if no crossing over all linked genes remain together during meiosis
-therefore passed onto gametes
-higher proportion of offspring will have parents genotype and phenotype
-closer together=more likely linked

Question 11

Crosses involving epistasis

Answer 11

Interaction of non-linked genes where one masks the
expression of the other

Question 12

Chi-sqaured

Answer 12

• Chi-squared test = statistical test to find out whether the
  difference between observed vs expected data is due to chance
• i.e. ‘goodness of fit’ of a hypothetical model
• When to use the chi-squared test:
• The data are in categories (i.e. discrete variation)
• The data indicate absolute numbers (frequencies not %)

Question 13

Limitation of Chi-squared

Answer 13

• Due to the random nature of gamete fusion, these are rarely 100% accurate predictions

Question 14

How do you do a chi-squared test?

Answer 14

1. Define null hypothesis
   - No significant difference between observed/expected data/frequencies
   - I.e. difference is due to chance
2. Calculation of chi-squared value
   Where O = frequencies observed
   E = frequencies expected
   = sum of
3. Determine the number of degrees of freedom
   (number of categories - 1)
4. Determine critical value at p = 0.05 (5% probability) from a table given

Question 15

How do you interpret the results of a Chi squared
test?

Answer 15

a. If calculated value of Chi-squared is larger than the critical value at p = 0.05
b. Hence there is less than 5% probability that the differences between the observed and expected data are due to chance i.e. difference is significant
c. Reject the null hypothesis
OR
a. If calculated value of Chi-squared is smaller than the critical value at p = 0.05
b. Hence there is more than 5% probability that the differences between the observed and expected data are due to chance i.e. not significant
c. Accept the null hypothesis