Inheritance

Question 1

Genotype

Answer 1

Genetic constitution of an organism

e.g. BB, Bb, bb (eye colour)

Question 2

Phenotype

Answer 2

The expression of the genotype and its interaction with the environment

e.g. brown eyes

Explains why genetically identical twins may have diverging phenotypes, especially as they get older and are exposed to more environmental stimuli

Question 3

Allele

Answer 3

Different versions of a gene as the sequence of bases are different

In a diploid organism, the alleles at a specific locus may be either homozygous or heterozygous

Alleles may be dominant, recessive or codominant

Question 4

Locus

Answer 4

The fixed position of a gene on a chromosome

Question 5

Homozygous

Answer 5

Both alleles at a specific locus on each homologous chromosome are the same

e.g BB or bb

Question 6

Heterozygous

Answer 6

Both alleles at a specific locus on each homologous chromosome are different e.g. Bb

Question 7

Dominant

Answer 7

In heterozygous state, its the allele that is expressed in the phenotype

Question 8

Recessive

Answer 8

In heterozygous state, its the allele that isn’t expressed in the phenotype

Only expressed if homozygous recessive

Question 9

Codominant

Answer 9

Both alleles for same gene in heterozygous organism contribute to phenotype

Question 10

Homozygote

Answer 10

An organism that carries two copies of the same allele

e.g. BB, bb

Question 11

Heterozygote

Answer 11

An organism that carries two different alleles

e.g. Bb

Question 12

Genetic diagrams

Answer 12

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

Contains parent phenotypes/genotypes, parent gamete genotypes, and Punnet square (offspring genotypes, labelled with phenotypes)

Question 13

Monogenic inheritance

Answer 13

Inheritance of a characteristic controlled by a single gene

e.g. inheritance of wing length in fruit flies

Question 14

Dihybrid crosses

Answer 14

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 independant segregation

e.g. possible genotypes of gametes from AaBb adult: Ab, AB, ab, aB

Question 15

Crosses involving codominance

Answer 15

Both alleles in a heterozygous individual contribute to phenotype

e.g. coat colour in cattle: a red and white allele will end up in a speckled coat as both alleles are expressed

Question 16

Crosses involving multiple alleles

Answer 16

Individuals only have 2 alleles of a gene (2 homologous chromosomes) but there may be more than 2 alleles in the population

There is usually a dominance hierarchy

Question 17

Crosses involving sex linkage

Answer 17

A gene is ‘sex-linked’ if its locus is on a sex-chromosome

Specific characteristics are more likely to be inherited in either male or female offspring

Genes are more likely to be X linked (found on X chromosome)

Females (XX) have two copies of the gene/allele and only express the recessive allele if homozygous recessive

Males (XY) have only have one copy of the gene/allele and it cant be heterozygous so the recessive X-linked allele can only be expressed if only one copy is present

Question 18

Crosses involving autosomal linkage

Answer 18

Two genes carried on same autosome (non-sex chromosome) is autosomal linkage

Genes stay together during independant segregation of chromosomes in meiosis I

Assuming there is no crossing over, all linked genes remain together during meiosis and therefore linked genes pass onto the gametes (and offspring) together

Because two autosomally linked genes are inherited together, a higher proportion of offspring will have parents genotype and phenotype

Closer together on autosome means they are more closely linked and are therefore less likely to split up during crossing over

Question 19

Crosses involving epistasis

Answer 19

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

Question 20

Chi-squared test what is it

Answer 20

Statistical test to find out whether the difference between observed and expected data is due to chance

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

Question 21

Chi-squared test when to use it

Answer 21

When the data is in categories (i.e. discrete variation)

When the data indicates absolute numbers (frequencies not %)

Question 22

Chi-squared test how to apply it

Answer 22

Define the null hypothesis

No significant difference between observed/expected data/frequencies i.e. difference is due to cahnce

Calculation of chi-squared value

Determine the number of degrees of freedom (number of categories - 1)

Determine critical value at 5%

Critical value greater than 5% → difference is significant so reject null hypothesis

Critical value smaller than 5% → difference is not significant so accept null hypothesis