5.4 Inheritance

• Biologie
• Middelbare school
• vwo
• Leerjaar 2

5.4 inheritance

allele: variety of one gene

Plural = alleles

on each chromosome you

have an allele of a gene.

For example eye colour:

blue allele or brown allele

gene is general for example a gene for eyecolor

the specific eyecolor information is in the allel

dominant - the allel that is expressed

recessive - gets masked by the dominant allel

Such a person has brown eyes, but we call him or her a carrier of blue eyes. 

Is it a gene or an allele?

GENE

ALLELE

skin colour

curly hair

flower colour

blue eyes

blood type

type A blood

colour-blindness

hair type

short wings in a fruit fly

Evie says: 'all humans have the same genes, but not the same alleles'.
Mila says: 'all humans have the same alleles, but not the same genes'.
Who is right?

1. Evie
2. Mila
3. They both are
4. They are both wrong

dominant and recessive

genotype is written down in letters.

for example

brown eye color A

blue eye color  a

genotype Aa    fenotype = brown eyes

dominant, recessive alleles

dominant A   is always visible in the genotype

recessive a  is only visible when there is no dominant allel

Dominant wins over recessive

Two equal alleles: homozygous (AA or aa)

Two different alleles: heterozygous (Aa)

AA  => homozygous dominant  => brown eyes (fenotype)

Aa => heterozygous => brown eyes (fenotype)

aa => homozygous recessive => blue eyes (fenotype)

Some fruit flies have curly wings instead of normal wings (see the pictures below). Answer the questions using the letter R for normal wings and r for curly wings:

normal wings                 curly wings

a. Which is the dominant allele? Give the letter and the characteristic.

b. Which is the recessive allele? Give the letter and the characteristic.

c. What is the genotype of a fruit fly that is homozygous for curly wings? 

d. Which two genotypes will result in the ‘normal wings’ phenotype?

e. Can a fruit fly with curly wings have a heterozygous genotype? Explain your thinking.

a) R normal wing

b) r curly wing

c) rr

d)Rr and RR

e) curly wings is recessive and so it will always be masked by a R allel. Thus, it can never be heterzygous.

mendelian inheritence

we can calculate probable percentages of offspring with different genotypes/fenotypes

we use a Punnet square

Steps with Punnet Squares

1. Identify your alleles.
2. Identify genotype parent 1. Show possible gametes.
3. Identify genotype parent 2. Show possible gametes.
4. Fill in your punnet squares.
5. Write down ratios of the possible genotypes.
6. Write down ratios of the possible phenotypes.

1 Punnett Squares  = predicating genetic crosses

Punnett squares - used to predict outcomes of crosses:

CALCULATE THE CHANCE. 

Check out the steps on page 136.  

Let's practice! A guinea pig is homozygous for blue fur. This guinea pig mates with a guinea pig with yellow fur. Use a punnet square to determine the probability of one of their offspring having blue fur.

ff blue fur

Ff yellow fur

ff x Ff

            F                f

f          Ff               ff

f          Ff               ff

50% blue fur

Practice

5.4

all exercises