Section 7 Genetic Inheritance

Genes Genes are parts of chromosomes that carry information Most organisms get half their genetic information from one parent and half from the other so different forms of the same gene are called alleles

Genotype and Phenotype Genotype describes the alleles present for a characteristic Genotypes are always described using letters e.g. Bb, bb, BB Phenotypes describe the outward physical appearance of a genotype and are always described using words e.g. Brown or blue eyes

Alleles Alleles can be dominant, recessive or co- dominant Dominant alleles show up in the physical appearance of an organism Recessive alleles do not show up in the physical appearance of an organism unless there are two of them present Co-dominant alleles both show up in the physical appearance

Alleles Two alleles that are the same is known as a homozygous genotype Further description can be added to see if it dominant or recessive AA would be homozygous dominant aa would be homozygous recessive

Allele Two alleles that are different are known as a heterozygous genotype No further description is needed here because in a heterozygous genotype there must be two forms of the same gene e.g. Aa

True Breeding Organisms which pass on characteristics in a predictable way are said to be true breeding True breeding organisms are homozygous

Word Test 1. Different forms of the same gene. 2. Describes the alleles present for a characteristic. 3. Describe the outward physical appearance of a genotype and are always described using words e.g. Brown or blue eyes. 4. Alleles show up in the physical appearance of an organism. 5. Alleles do not show up in the physical appearance of an organism unless there are two of them present. 6. Alleles both show up in the physical appearance. 7. Two alleles that are the same. 8. Two alleles that are different. 9. Organisms which pass on characteristics in a predictable way.

True Breeding Monohybrid Cross

Predictions Vs Reality With true breeding parents, the expected ratio in the F2 generation is normally 3:1 In reality, the expected numbers differ from the actual numbers This is because fertilisation is a random process

Co-dominance Sure, why not Red Cow RR White Cow WW Roan Cow RW

Co-dominance Two alleles of a gene can be co-dominant when neither is dominant nor recessive. Both alleles can be expressed E.g. red cows crossed with white will generate roan cows. Roan refers to cows that have red coats with white blotches. When the F1 roan are self crossed, the F2 have a phenotypic ratio of : 1 red:2 roan:1 white. This type of inheritance is called incomplete dominance.

Make a baby! The way people look is all down to random chance. Having dominant, recessive or co-dominant characteristics is totally dependant on which egg and sperm meet at fertilisation and which genes these gametes were allocated during meiosis. Today you are going to pair up with someone and “make a baby” by carrying out the following instructions: ◦ Traits will be written on the board – the dominant, recessive or co- dominant version will be described. ◦ You cannot choose which trait your baby has, this will be determined by rolling the dice. If you have an even number, it is dominant, odd numbers are recessive. ◦ Work through all the characteristics and draw your baby. Have a look at other couples’ babies. Do they look similar or different?!

Family Trees P F1 F2

Polygenic Inheritance Poly = many Genic = genes “many gene” inheritance Two or more genes control a characteristic This means there are lots of different phenotypes Examples include: hand span, height, weight, foot length, leaf length

Continuous Variation Continuous variation is when there are no clear cut distinctions between groups E.g. Weight: some people are 8st 1lb, some are 9st 12lb There are NO distinct groups e.g. 8st or 9st etc CONTINUOUS VARATION = POLYGENIC INHERITANCE

Normal distribution graphs Continuous variation can be measured and plotted on a line graph or histogram:

Examples of continuous variation include: Height Weight Hand span Length of feet Milk yield in cows

Single Gene Inheritance Characteristics are controlled by just one gene There are therefore only one of two phenotypes E.g. Tongue rolling or non-tongue rolling, green or yellow seeds

Discontinuous Variation There are clear-cut differences between groups E.g. Being able to roll your tongue or not; you can either do it or you cannot SINGLE GENE INHERITANCE = DISCONTINUOUS VARIATION

Bar graphs Discontinuous variations are shown using bar graphs

Blood type