Biology - 7.1 Inheritance

What is meant by the term genotype?

Genetic constitution of an organism (complete set of genetic material)

What is meant by the term phenotype?

The expression of the genotype (genetic constitution) and its interaction with the environment

What are alleles and how do they arise?

• Variations of a particular gene (same locus)

• Arise by mutation (changes to DNA base sequence)

How many alleles of a gene can be found in diploid organisms?

Two

• Have 2 sets of organisms (cromosomes found in homologous pairs)

• May be more (than 2) alleles of a single gene in a population

Describe the different types of alleles

Dominant

• Always expressed (shown in the phenotype)

Recessive

• Only expressed when 2 compies present (homozygous recessive)/ not expressed when dominant allele present (heterozygous)

Codominant

• Both alleles expressed / contribute to phenotype (if inherited together)

What is meant by homozygous and heterozygous?

Homozygous

• Alleles at a specific locus (on each homologous chromosome) are the same

Heterozygous

• Alleles at a specific locus are different

What do monohybrid and dihybrid crosses show?

Monohybrid cross

• Inheritance of one phenotypic characteristc coded for by a single gene

Dihybrid cross

• Inheritance of two phenotypic chrarcteristics coded for by two different genes

Explain the evidence from a pedigree diagram which would show an allele for (phenotype) is dominant

• (phenotype) parents (n+n) have child (n) without (phenotype)

• So bothe parents (n+n) must be heterozygous / carriers of recessive allele

  • If it were recessive, all offspring would have (phenotype)

Explain the evidence from a pedigree diagram which would show an allele for (phenotype) is recessive

• Parents (n+n) without (phenotype) have child (n) with (phenotype)

• So both parents (n+n) must be heterozygous / carriers of recessive allele

What is a sex-linked gene?

A gene with a locus on a sex chromosome (normally x)

Explain why males are more likely to express a recessive X-linked allele

• Assuming males are XY and females XX, as in humans

  • If not females XY more likely

• Females XX have 2 alleles → express recessive if homozygous recessive / can be carriers

• Males XY have 1 allele (inherited from mother) → recessive allele always expressed

Explain the evidence from a pedigree diagram which would show that the allele for (phenotype) on the X-chromosome is recessive

• Mother (n) without (phenotype) has child (n) with (phenotype)

• So mother (n) must be heterozygous / carrier of recessive allele

Explain the evidence from a pedigree diagram which would suggest (recessive phenotype) is caused by a gene on the X

Only males tend to have (recessive phenotype)

Explain the evidence from a pedigree diagram which would show that the gene for (phenotype) is not on the X-chromosome

• (Phenotype) father (n) has daughter (n) without (phenotype)

• Father (n) would pass on allele for (phenotype) on X chromosome so daughter (n) would have (phenotype)

OR

• (Phenotype) mother (n) has son (n) without (phenotype)

• Mother (n) would pass on allel for (phenotype) on X chromosome so son (n) would have (phenotype)

ASSUMES females are XX and males are XY like humans, if not - swaps sons with daughters + fathers with mothers

Explain how autosomal linkage affects inheritance of alleles

• Two genes located on same autosome (not-sex chromosomes)

• So alleles on same chromosome inherited together

  • Stay together during independent segregation of homologous chromosomes in meiosis

• But crossing over between homologous chromosomes can create new combinations of alleles

  • If genes are closer together, they are less likely to be split by crossing over

Describe when a chi-squared (x²) test can be used

• When determining if observed results are significantly different from expected results (frequencies)

• Data is categorical (divided into groups like phenotypes)

Suggest why in genetic crosses, the observed phenotypic ratios obtained in the offspring are often not the same as expected ratios

• Fusion / fertilisation of gametes is random

• Autosomal linkage / sex-linkage

• Small sample size → not representative of whole population

• Some genotypes may be lethal / incompatible with life

Described how chi-squared can be calculated

O = observed frequencies

E = expected frequencies

Sum of (O-E)² / E

Describe how a chi-squared value can be analysed

1. Number of degrees of freedom = number of categories -1

2. Determine critical value pf p=0.05 (5% prob) from a table

3. If x² value is (greater/less) than critical value at p=0.05

   • Difference (is/not) significant so (reject/accept) null hypothesis

   • So there is (less/more) than 5% probability that difference is due to chance