Human variation & inheritance

What are some factors that increase genetic variation?

• Mutations

• Independent assortment & crossing over during meiosis

• Random fertilisation during sexual reproduction

What is a mutation?

A change in the number, or sequence, of bases in a particular gene

What is the definition of a gene?

A sequence of nucleotide bases that encodes an amino acid sequence of a polypeptide

Why might a mutation not lead to change in the amino acid sequence?

The genetic code is degenerate, meaning the mutation may end up coding for the same amino acid as the original triplet unless a frame shift occurs

What are addition & deletion mutations?

One or more nucleotides (bases) are either inserted or deleted from the DNA sequence:

• this is more likely to be harmful & significant, as it leads to a frame shift, meaning the entire amino acid sequence downstream of the mutation will be different

What is a substitution mutation?

A nucleotide in the DNA sequence is replaced by another:

• this is more likely to be a silent mutation, meaning no change occurs in the amino acid sequence

What is crossing over & how does it produce genetic variation?

• While paired in a bivalent during prophase I, chromatids of homologous chromosomes swap alleles/genes at the chiasmata

• This produces new combinations of alleles

What is independent assortment & how does it produce genetic variation?

• Homologous pairs of chromosomes align randomly during metaphase I, leading to a random movement to the poles of the cell

• This leads to different combinations of chromosomes in gametes

How does random fertilisation bring about genetic variation?

• Gametes are haploid cells, meaning they only contain half of a person’s DNA

• As this is determined by meiosis, every gamete contains different DNA

• Therefore, the same two individuals can produce genetically different offspring

What is a translocation mutation?

• One part of a chromosome is swapped with a part of a different chromosome

• Swapped parts can be unequal in size & between non-homologous chromosomes

What is a non-disjunction mutation?

• Chromosomes fail to separate correctly during anaphase in meiosis

• Gametes may end up with one extra copy of a particular chromosome or no copies of a particular chromosome

How can non-disjunction lead to polysomy?

If a gamete with an extra chromosome is fertilised by a normal gamete, the zygote will show trisomy:

• three chromosome copies, instead of two (e.g. Down’s syndrome, which is caused by trisomy 21)

How can non-disjunction lead to monosomy?

If a gamete with a missing chromosome is fertilised by a normal gamete, the zygote will show monosomy:

• one chromosome copy, instead of two (e.g. Turner’s syndrome, which is X0)

What is the definition of genotype & phenotype?

• Genotype: the combination of alleles an organism possesses

• Phenotype: the observable characteristics of an individual, determined by both genotype & environment

What is an allele?

Different forms of a particular gene, found at the same locus (position) on a chromosome

What is the definition of dominance & recessive?

• Dominance: a trait which is present if an individual has at least one copy of the allele

• Recessive: a trait which is only present when an individual has two copies of the allele & can be masked by a dominant allele

What is the definition of codominance & multiple alleles?

• Codominance: two dominant alleles for a gene in a heterozygous organism equally contribute to the phenotype

• Multiple alleles: a gene with more than two alleles which can code for a characteristic

What is the definition of homozygote & heterozygote?

• Homozygote: an organism which has two of the same versions of a gene (both alleles are dominant or recessive)

• Heterozygote: an organism which has two different versions of the same gene (one allele is dominant, the other is recessive)

Draw a genetic cross diagram for two heterozygous parents with blue eyes & determine the offspring’s ratio:

• (B) = blue allele

• (b) = white allele

What is a pedigree diagram?

Used to trace the pattern of inheritance of a specific trait through generations of a family → provides information, such as:

• whether a trait is caused by a dominant or recessive allele

• whether a trait is more likely to be inherited by males or females

• the genotype of individuals in the family

• the probability that an individual in the family will inherit a trait

How are non-interacting, unlinked genes inherited?

Can be monohybrid or dihybrid:

• monohybrid = a phenotypic characteristic is controlled by a single gene

• dihybrid = two phenotypic characteristics are controlled by two different genes present on two different chromosomes

What is meant by autosomal linkage?

Two or more genes are located on the same, non-sex chromosome:

• only one homologous pair is needed for all four alleles to be present

• for genes that aren’t linked, two homologous pairs are needed

Referring to meiosis, explain why autosomally linked genes are inherited in pairs

• Genes on the same chromosome, particularly those closest together, are unlikely to undergo recombination (independent assortment & crossing over) during meiosis

• Therefore, they are inherited as if they were the same gene

What is meant by sex-linkage & what is an example?

An allele is located on one of the sex chromosomes, meaning its expression depends on the sex of the individual (e.g. haemophilia)

Why are males more likely to express a recessive sex-linked allele than females?

• Most sex-linked alleles are located on the X chromosome

• Therefore, males only get one copy of the allele (XY), so will express this characteristic even if it’s recessive

• Since females get two alleles (XX), this is less likely