Keywords

    Independent assortment: Alleles of genes on non homologous chromosomes assort independently during gamete formation.

Segregation: The two alleles for each gene (on homologous chromosomes) separate during gamete formation.

Linkage: Genes which are carried on the same chromosomes tend to be inherited together. Crossing over may separate linked genes.

Chromosome map: Shows the genes on one chromosome and how close together they are.

Which is the odd one out and why?

Independent Assortment Chromatid Crossing Over Chromosome mutation

6. Linked genes and gene maps

SC: Explain how linkage may affect phenotypes shown in offspring Describe what is shown by a gene map.

Benefici al / Harmful Where, when , how?

Mutations pg 93 Sources of variation Gene Pools pg 92 Sexual Reproduction Linkage pg 103 Segrega -tion pg 99 Independ ent assortment pg 101 Crossing over pg 102 Genetics Tool Box – Intro.

Genetic Variation and Change Mendelian Genetics Pg 109 Using monohybrid crosses Incomplete dominance pg 109 Lethal alleles pg 110 Multiple alleles pg 110 Co dominan ce pg 109 Using dihybrid crosses pg 113 Indepen dent assortmen t pg 114 Linkage pg 118 Population Genetics pg 123 Genetic Drift pg 126 Changes to allele frequencies Migratio n pg 126 Natural selectio n pg 132

Independant assortment recap

In Labradors black(B) is dominant to brown coat(b) and blindness (n) is recessive to normal vision (N).

As these two genes are on different chromosomes they move independently into gametes depending on how they line up along the equator.

If two dogs were bred which were heterozygous for each trait what possible gametes could form?

What percentage of each gamete should form?

Did you know?



The number of genetic variations possible in our sperm and eggs thanks to independent assortment is 2 crossing over or mutation.

23 or 8388608 – that is before we add in variation due to

Linked genes

     Are genes which are on the same chromosome.

They do not show independent assortment unlike genes on different chromosomes. During Anaphase two, these genes travel to the same gamete together.

Linked genes have different phenotype ratios in gametes than genes on different chromosomes.

Linked genes only become separated if crossing over occurs between the two gene locii.

Gene Maps

 1.

2.

3.

Tell us three things… What genes are linked What genes are on the chromosome How close genes are to each other and the likelihood of them being separated by crossing over.

See the sample human gene map.

Sex Linkage

A sex linked condition on the Y chromosome will not show in females. XY’ X linked recessive traits show up more in males as they only have one X chromosome so if they gain that allele from their mother, they will have that trait. X’Y For a female to show an X linked trait they must have two copies of the allele. X’X’ Examples of sex linked traits: haemophilia, red green colour blindness, muscular dystrophy

Example Exam Question

Discuss how and why the different processes that can occur during meiosis can lead to genetic variation between individuals.

Take into account the following: • independent assortment • segregation • recombination / crossing over.