Section 7Section 7Genetic Inheritance

GenesGenesGenes 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 Genotype and PhenotypePhenotypeGenotype 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

AllelesAllelesAlleles 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

AllelesAllelesTwo 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

AlleleAllele

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 BreedingTrue Breeding

Organisms which pass on characteristics in a predictable

way are said to be true breeding

True breeding organisms are homozygous

Word Test1. 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 True Breeding Monohybrid CrossMonohybrid Cross

True Breeding True Breeding Monohybrid CrossMonohybrid Cross

Predictions Vs RealityPredictions Vs RealityWith 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-dominanceCo-dominance

Sure, why not

Red CowRR

White CowWW

Roan CowRW

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 TreesFamily Trees

P

F1

F2

Polygenic InheritancePolygenic InheritancePoly = manyGenic = 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 VariationContinuous VariationContinuous 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 graphsContinuous 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 Single Gene InheritanceInheritanceCharacteristics 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 Discontinuous VariationVariationThere 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

number of plants

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white pink red purple

Flower colour

Nu

mb

er o

f p

lan

ts

Discontinuous variations are shown using bar graphs

Blood type