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Refresh of Mendelian Genetics

Who was Gregor Mendel?

He was monk living in a monastery in Czechoslovakia in mid-1800’s.

He was in charge of the monastic vege garden.

He did breeding expt on peas. He cultivated ~29,000 peas

plants!!…..his data was meticulously

recorded. The amount of data is what

makes his findings so reliable.

Thanks to Mendel….We know about words like…. Homozygous dominant and recessive Heterozygous Genes Alleles Genotype Phenotype Mendel’s laws….

YOU WOULD HAVE LEARNT THESE TERMS LAST YEAR

To see what you can remember....Match and paste...

Chromosome structure◦ Gene – unit of inheritance – segment of DNA

Alleles◦ 2 versions of a gene (alleles)◦ Dominant, recessive◦ Alleles separated in the formation of gametes◦ Each gamete carries one allele for each pair◦ At fertilisation there is random uniting of gametes

Review

Monohybrid crosses You learnt about these in Y11 science….

They are genetic crosses of just one trait.

Punnet squares have 4 squares.

An individual that displays a dominant trait can either be homozygous dominant or heterozygous

◦ Eg. Straight thumb (H) is dominant to hitchhikers thumb (h)How do we tell if a straight thumbed individual is

homozygous dominant or heterozygous?

Test Cross

Tongue rolling (T) is dominant to non-tongue rolling (t). A tongue roller and non-tongue roller had children and produced tongue rollers and non-tongue rollers as children.

What were the genotypes of the parents?

Draw a punnet square to show possible off spring combinations.

DO NOW

Sometimes alleles are not completely dominant over another allele.

Example: A red snapdragon flower crossed with

a white flower could create a pink offspring. F1 are all heterozygous. Self-crossed F2 yields a ratio of:

1 : 2 : 1Red: Pink: White

Incomplete Dominance

The heterozygous genotype shows both traits.

Example:BB = Black spotted cat

bb = orange spotted catBb = Black and orange spotted cat

Ratio 1 : 2 : 1 Black : Black+Orange : Orange

BLOOD GROUPS ALSO SHOW CODOMINANCE

Codominance

Alleles which cause death when occurring as one of the homozygous genotypes

These cause death at embryo stage and so the ratio is 1:2

Example: Achondroplastic dwarfism… 1 : 2 : 1 AA : Aa : aaDied:Dwarf:Normal

Lethal Alleles

Dwarfism is a result of autosomal dominant mutation in the fibroblast growth factor receptor gene 3 (FGFR3), which causes an abnormality of cartilage formation. In normal circumstances, FGFR3 has a negative regulatory effect on bone growth. In achondroplasia, the mutated form of the receptor is constitutively active and this leads to severely shortened bones.

Types of Explanations

More than one allele can be positioned at a gene loci (address)

Example: Blood groups in people…..The protein's structure is controlled by three

alleles; i, IA and IB. The first allele is, i, the recessive of the three, and IA and IB are both co-dominant when paired together

Multiple Alleles

A and B are co-dominant and both are dominant to O….so O is recessive.

I is the allele that makes the antigen.

The antibody causes foreign antigens to clump together.

Explanation

Wool Fibre Diameter – B2A gene◦ A finest wool◦ B◦ C thickest wool

Another wool fibre gene KRT 1.2 ◦ Wool protein◦ Many alleles

Multiple alleles

A cross between two organisms where the inheritance patterns of TWO GENES are studied

There are a greater number of gamete types produced when two genes are considered

The genes are carried by separate chromosomes and are sorted independently of each other (WE WILL COVER THIS POINT SOON!)

Dihybrid Cross

Answer questions◦ Dihybrid Easy◦ More difficult dihybrid◦ Problems in

Dihybrid Crosses

Definitions

Back of books 1-10

Genes located on one sex chromosome but not on the other are called sex-linked genes.

The traits that show this kind of inheritance are almost always carried on the X chromosome

Examples: red-green colour blindness, haemophilia,

Sex Linkage

Thought to be Y linked!!

For males, any sex linked gene on the X chromosome will show up because the Y has no matching gene to mask the effect

In females (having two X’s) the effect usually is masked – however they are ‘carriers’

(we will make sense of this statement!!)

Sex-Linked Genes

A woman with normal vision whose father was colour- blind marries a man who is colour blind. Give the possible phenotype and genotype of such a cross.

C = normal c = colour blind

Example

Research the genetics of tortoisehell cats and hand to me tomorrow on a separate piece of paper

Homework

1. Mendel’s law of segregation – ‘of the two genes controlling each characteristic, only one is present in each gamete’.

2. Mendel’s law of Independent Assortment – ‘the segregation of one pair of alleles does not affect the segregation of another pair’

Mendels Laws

The Law of Segregation states that the members of each pair of alleles separate when gametes are formed. A gamete will receive one allele or the other.

Mendels Laws

Law of Segregation the pair of alleles of each

parent separate and only one allele passes from each parent on to an offspring

which allele in a parent's pair of alleles is inherited is a matter of chance 

segregation of alleles occurs during the process of gamete formation (meiosis)

randomly unite at fertilization

The Law of Independent Assortment: As chromosome pairs line up, each chromosome pair is sorted independently. Which chromosome of each pairs ends up in which cell is random = produces different combinations of parents chromosomes in gametes.

The Law of Segregation states that the members of each pair of alleles separate when gametes are formed. A gamete will receive one allele or the other.

Law or Independent Assorment

During meiosis the homologous chromosomes line up at the equator of the spindle in pairs during Metaphase 1.

How they line up provides the basis for Independent Assortment.

Law of Independent Assortment

http://www.sumanasinc.com/webcontent/animations/content/independentassortment.html

Independent assortment

2 chromosome pairs alignment

During the process of meiosis, independent assortment and crossing over (recombination) can occur. This results in genetic variation in the offspring of sexually reproducing individuals

◦ Describe what happens during independent assortment

◦ Explain how crossing over (recombination) can contribute to the genetic variation that results from sexual reproduction. You may wish to draw a diagram.

◦ Genetic variation can be a results from mutation. Explain the results of mutation sin somatic and gametic cells

Exam question(2008)