5.2- patterns of inheritance and variation

Gene

A sequence of bases on a DNA molecule that codes for a protein

allele

Different versions of the same gene that code for variants of a characteristic

genotype

An organism's genetic makeup - its alleles

phenotype

An organism's physical characteristics as determined by its genotype and environment

dominant

An allele that is always expressed in the phenotype, even when only one copy is present in the genotype

recessive

An allele that is only expressed when it is homozygous in the genotype, and is masked by a dominant allele

locus

the specific position of a gene on a chromosome

homozygous

An organism with two identical alleles for a trait is homozygous

Heterozygous

An organism with two different alleles for a trait is heterozygous

Monohybrid inheritance

• involves the transmission of one gene from parents to their offspring

• A monohybrid cross tracks the inheritance pattern of the alleles for a single characteristic, controlled by one gene with two possible alleles.

monohybrid cross

tracks the inheritance pattern of the alleles for a single characteristic, controlled by one gene with two possible alleles

F1 generation

• the offspring from a cross between individuals with homozygous dominant and homozygous recessive genotypes.

• All the offspring will be heterozygous, inheriting a recessive allele from one parent and a dominant allele from the other

F2 generation

• the offspring from a cross between two heterozygous F₁ individuals

• It results in offspring exhibiting a 3:1 ratio of dominant to recessive phenotypes

codominance

• occurs when two different alleles are equally expressed in an organism's phenotype. Unlike typical dominant-recessive gene relationships, both alleles in codominance share equal dominance

Multiple alleles

• refer to genes that exist in more than two allelic forms. However, an individual can only have two alleles of a specific gene at any one time.

effect of multiple alleles

increases the phenotypic diversity among individuals in a population

example of multiple alleles

The ABO blood groups

• This system is controlled by a gene known as the immunoglobulin, or I, gene

how many pairs of chromosomes do humans have?

23

autosomes

chromosomes that do not determine the sex. Humans have 22 pairs

sex chromosomes

x and y

x chromosome

found in both males and females

Y chromosome

found in only males

sex-linked genes

Genes located on the X or Y chromosomes

why are most sex-linked genes x-linked?

The X chromosome carries the majority of these genes due to its larger size

How do recessive alleles on the X chromosome tend to appear?

in the phenotype more often in males, as there is no corresponding allele on the Y chromosome to mask them

Haemophilia

an X-linked recessive disorder, which is one that is caused by a defective gene on the X chromosome

how is haemophilia caused?

a recessive X-linked allele that alters the DNA sequence coding for a crucial blood clotting protein. This faulty allele can cause reduced blood clotting, leading to excessive bleeding.

Dihybrid cross

shows the simultaneous inheritance of two genes controlling separate characteristics

what can dihybrid crosses be used for?

1. Determine whether genes are linked.

2. Locate genes on specific chromosomes.

3. Calculate expected phenotypic ratios in subsequent generations.

law of independent assortment

alleles for different genes segregate independently during gamete formation in meiosis, unless they are linked

why do actual observations sometimes differ from predictions of phenotypic ratios?

Random fertilisation:

• Gamete fusion is a chance process.

Linked genes:

• Linked genes are on the same chromosome so alleles are usually inherited together, maintaining the parents’ original allele combinations in offspring.

• However, crossing over during meiosis can sometimes change these allele combinations by separating linked genes.

autosomal linkage

When genes are located on the same autosome

• Linked genes tend to be inherited together in offspring, rather than assorting independently

effects of crossing over on autosomal linkage

• Crossing over potentially separates linked genes.

• However, when genes are linked, fewer recombinant offspring tend to be produced.

• This indicates less genetic variation being introduced from crossing over when genes are linked.

• the closer the genes, the more likely they will be inherited together.

recombinant offspring

offspring whose allele combinations are different from those found in either parent, due to crossing over or independent assortment during meiosis

formula for recombination frequency

number of recombinant offspring/total number of offspring