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.

6. Linked genes and gene maps SC: Explain how linkage may affect phenotypes shown in offspring Describe what is shown by a gene map. Which is the odd one out and why? Independent Assortment Chromatid Crossing Over Chromosome mutation

Genetic Variation and Change Gene Pools pg 92 Sources of variation Mutations pg 93 Sexual Reproduction Segrega -tion pg 99 Independ ent assortment pg 101 Crossing over pg 102 Linkage pg 103 Benefici al / Harmful Where, when, how? Mendelian Genetics Pg 109 Population Genetics pg 123 Changes to allele frequencies Using monohybrid crosses Natural selectio n pg 132 Migratio n pg 126 Genetic Drift pg 126 Lethal alleles pg 110 Using dihybrid crosses pg 113 Co- dominan ce pg 109 Multiple alleles pg 110 Incomplete dominance pg 109 Indepen- dent assortmen t pg 114 Linkage pg 118 Genetics Tool Box – Intro.

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 23 or – that is before we add in variation due to crossing over or mutation.

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  Tell us three things… 1. What genes are linked 2. What genes are on the chromosome 3. 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.