Genetics and Inheritance Alleles and Appearance. Diploid zygote Haploid gametes

Genetics and Inheritance

Alleles and Appearance


Diploid zygote

Haploid gametes


Remember: A gene is part of a chromosome. Each inherited characteristic is controlled by 2

copies of its gene. The 2 copies of the gene that determine a

particular characteristic may be the same or different.

Different forms of the same gene are called alleles.


Remember that each gene occupies a specific site on a chromosome.

Consider the gene for height in pea plants.

Allele for ‘tall’ Allele for ‘dwarf’

What will the appearance of the pea plant be?

Alleles and Appearance Note

Alleles are described as dominant or recessive.

A dominant allele always shows up in an organisms appearance (masks the recessive allele).

A recessive allele only shows up if there are two of them.


Alleles are symbolised using lettersDominant alleles are given capital letters,

while recessive alleles are given the same letter but in lower case.

For example, in pea plants the dominant allele for tallness has the symbol ‘T’ and the recessive allele for dwarfism has the symbol ‘t’.


Dwarf = Recessive (t)Tall = Dominant (T)

T T T tt t

Tall TallDwarf

Alleles and Appearance Notes

Collect and complete the cut-out “Symbols for Alleles”, by choosing suitable symbols (letters) for the alleles.


The two descriptions of an organisms characteristics that you need to know are:

Genotype

and

Phenotype


The genotype of an organism is the alleles it carries for a particular characteristic. Genotypes are usually written as letter symbols, e.g. Tt.


The phenotype of an organism is the outward appearance of one of it’s characteristics and is usually described in words, e.g. tall.

Phenotype is dependent upon genotype.


More vocabulary!

A homozygous organism has two identical alleles of a gene, e.g. TT

A heterozygous organism has two different alleles of a gene, e.g. Tt


Collect and complete the cut-out “Genotypes and Phenotypes in Mice”

then

Collect and complete the cut-out “Predicting Phenotypes”. In fruit flies, the grey body allele (G) is dominant to the black body allele (g).


Think:

Can different genotypes give the same phenotype?

Can the same genotype give different phenotypes?


The monohybrid cross

The Monohybrid Cross Notes

A monohybrid cross is a breeding experiment which follows the inheritance of just one characteristic, for example coat colour in mice.


Organisms which are true breeding pass on the same characteristics to their offspring over many generations.

This means that the offspring of true breeding black mice are always black and the offspring of true breeding white mice are always white.

The Monohybrid Cross

Think!

Will the genotype of a true breeding organism be homozygous or heterozygous?


Let’s look at the example of mice.

In mice, black coat (B) is dominant to white coat (b).


XP (parents)

XF1 (first filial generation)

F2 (second filial generation)

Filial =

son / daughter


XP

XF1

F2

BB

BB BB

BBBBBBBB

BB

Gametes all ‘B’

Gametes all ‘B’


What would the genotype of true breeding white mice be?

What would be the genotype and phenotype of the F1 and F2 generation?


All offspring (F1) bb

bb bbParents x

bb bbx

Second generation (F2)

bb

Gametes all b b

Gametes all b b


XP

XF1

F2


Another type of cross is when each parent is a true breeding individual, but each has a different phenotype.

What would happen if you crossed a true breeding black mouse with a true breeding white mouse?


XP

F2

XF1

bb BB

BbBb

BbBB bbBb

BBbb

b bBB


This can be shown in a punnet square:

B b

B BB Bb

b Bb bb

GametesG

amet

es


Practice crosses:

1) Pea plants can be tall (T) or dwarf (t).

If a true breeding tall plant was crossed with a true breeding tall plant what would the phenotype and genotyope of the F1 and F2 generations be?


2) Pea plants can be tall (T) or dwarf (t).

If a true breeding tall plant was crossed with a true breeding dwarf plant what would the phenotype and genotyope of the F1 and F2 generations be?


3) R = red flowers r = yellow flowers

A plant homozygous for red flowers is crossed with a plant that has yellow flowers.

Work through the cross to the F2 generation.

What is the phenotype and genotype of the F1?

What is the phenotype and genotype ratios of the F2?


4) R = red flowers r = yellow flowers

A plant heterozygous for red flowers is crossed with a plant that has yellow flowers.

Work through the cross to the F1 generation.

What is the phenotype and genotype ratios of the F1?


There is often a difference between the observed and predicted numbers of different types of offspring.

This is because fertilisation is a random process involving the element of chance.


How can we tell if an organism is true breeding?

If a mouse has a black coat, it’s genotype could beBB – true breeding, orBb – not true breeding.


A test cross is used to identify the genotype of an individual showing a dominant characteristic by crossing it with a homozygous recessive individual.


BB or Bb?


B

B

b b

Bb

Bb

Bb

Bb

All offspring: black

B

b

b b

Bb

bb

Bb

bb

Ratio = Black : White 1 : 1

X

BB bb

X

Bb bb


Now read Tp189-190 ‘Human Inheritance’ and complete the Activity on p190-191.


Co-dominance

Co-dominance Notes

Co-dominance is a pattern of inheritance where both alleles are expressed equally.

Offspring from a cross between two true-breeding parents have a phenotype in between each parent.

Co-dominance – Think!

In horsesB = black coatW = white coat

What would be the phenotype of a horse with the following genotypes:BB?WW?BW?

Co-dominance and Blood Group

Blood group is determined by three alleles:A, B and O

A and B are co-dominant to one another Both are completely dominant to O

There are four blood group phenotypes:A, B, AB and O.

Co-dominance Notes

A man with blood group A whose father was blood group O marries a woman with blood group AB.Draw a punnet square to show all the

possible genotypes that their children could have.

Which phenotype could not occur among their offspring?


Polygenic Inheritance

Polygenic Inheritance

Variations between individuals fall into two categories:discontinuous or continuous

Discontinuous characteristics fall into distinct categories – examples?

Continuous characteristics show a range of differences from one extreme to the other – examples?

• Discontinuous:

• Controlled by alleles of a single gene

• Single-gene inheritance

Tongue rolling

Roller Non-roller

Blood Groups

O B A AB

• Continuous:

• Controlled by alleles of two or more genes

• Polygenic inheritance

Height Skin Colour

Discontinuous data Continuous data

No. of pupils

Tongue rolling ability

Roller Non-roller

Distinct data can be presented as a bar graph.

Pea seed shoot lengths (mm)

No. of seeds

1-5 6-10 11-15 16-20 21-25

Ranges of data can be presented as a histogram or

line graph.

Polygenic Inheritance Notes

A characteristic that is controlled by more than one gene and is expressed as a range of phenotypes is said to show polygenic inheritance. Include an example!


Organisms in genetic experiments

Organisms in genetic experiments

Qualities of an organism which make it suitable for genetic experiments are:Short life cycle, grow quicklyEasy to breed, produce many offspringEasy to observe contrasting characteristics

Drosophila

Drosophila (fruit flies) are suitable for carrying out genetic experiments as theyare very smallare easy to breedproduce large numbers of offspringhave a short life cyclehave many different phenotypes, e.g. body

colour are easy to distinguish, e.g. males have

rounded abdomens, females have pointed.

Using Drosophila Notes

The steps involved in setting up a cross to study the colour of the abdomen in Drosophila flies

Steps Notes

1. Parents are chosen, one with a grey body and the other with an ebony body.

Parents must be true breeding to ensure dominant alleles are not hiding recessive alleles.


Steps Notes

2. Breed the parents. This produces the F1 generation.


Steps Notes

3. The F1 phenotype is observed.

This identifies the dominant allele.


Steps Notes

4. The F1 generation is self-crossed.

This produces the F2 generation.


Steps Notes

5. The F2 generation is examined and the number of each phenotype counted.

This allows the phenotype ratio to be calculated.


Steps Notes

6. The experiment is repeated.

This improves the reliability of the results.

Documents

Genetics and Inheritance Alleles and Appearance. Diploid zygote Haploid gametes