Genetic Diagrams

Name all the Genetic Diagrams:

- Monogenic inheritance

- Dihybrid inheritance

- Multiple alleles

- Sex linkage

- Codominance

What is a monogenic cross?

A genetic cross that involves only 1 gene

Monogenic inheritance: F1 Generation

Parental Genotypes: RR x rr

Parental Phenotypes: Round Wrinkled

Gametes produced: (circled) R R r r

F1 Genotype: (Draw a punnet square) Rr, Rr, Rr, Rr

F1 Phenotype: Round, round, round, round

Monogenic inheritance: F2 Generation

Parental Genotypes: Rr x Rr

Parental Phenotypes: Round Round

Gametes produced: (circled) R r R r

F1 Genotype: (Draw a punnet square) RR, Rr, Rr, rr

F1 Phenotype: Round, round, round, wrinkled

The F2 generation produces a 3:1 phenotypic ratio of the dominant trait (R - Round) to the recessive trait (r - wrinkled)

There is a genotypic ratio for 1:2:1 for RR:Rr:rr

What is a dihybrid cross?

A dihybrid cross is used to show the inheritance of 2 different genes, each with 2 alleles.

Dihybrid inheritance: What's the difference between a dihybrid cross to a monohybrid cross?

A cross is in a similar format to the monohybrid cross, but in a dihybrid cross it has 4 alleles, not 2

Dihybrid Inheritance Pea Example:

Y (dominant) = Yellow

y (recessive) = Green

R (dominant) = Round

r (recessive) = Wrinkled

Complete a cross between a double homozygous dominant pea (YYRR) and a double homozygous recessive pea (yyrr), commenting on the ratio of phenotypes achieved.

YYRR x yyrr

Gametes: (circled) YR yr

F1 Genotype: (Draw a punnet square 2 by 2) YyRr

F1 Phenotype: 100% Yellow Round seeds

Show a cross between 2 peas from the F1 generation, commenting on the ratio of phenotypes created.

YyRr x YyRr

Gametes: (circled) YR, Yr, yR, yr and YR, Yr, yR, yr

F2 Genotype: (Draw a punnet square 4 by 4)

F2 Phenotype:

9 Yellow Round = 1 YYRR, 2 YYRr, 2 YyRR, 4 YyRr

3 Yellow Wrinkled = 1 YYrr, 2 Yyrr

3 Green Round = 1 yyRR, yyRr

1 Green wrinkled = 1 yyrr

9:3:3:1

Under usual conditions, what ratio will you always get with dihybrid crosses?

9:3:3:1

Why wouldn't you get the 9:3:3:1 ratio in a dihybrid cross?

Something may have gone wrong, like epistasis, or if the double homozygous recessive (the 1 in 9:3:3:1) is disadvantages and inherits a disease and does not survive.

Multiple alleles

- A gene is a section of DNA that determines a specific characteristic

- Most genes have 2 alleles, one inherited from each parent

- However, some genes have multiple alleles. This means the genes have more than just 2 possible alleles.

- Even if a gene has more than 2 possible alleles overall, an individual can only carry 2 alleles at a time (one on each of their 2 homologous chromosomes)

Multiple alleles: Blood

Sex Linkage

Sex linkage means the gene for a characteristic is found on a sex chromosome.

Boys

XY

Girls

XX

If a characteristic caused by a recessive allele on the X chromosome (which is missing in the Y chromosome), will boys or girls more likely express it?

Boys, because they only have XY chromosomes. They cannot be carriers of a recessive disorder.

Girls have XX chromosomes, and the other X chromosome usually has the dominant allele.

Name a sex linked genetic disorder

Haemophilia

Why is haemophilia more prevalent in men?

A recessive allele codes for haemophilia, on the X chromosome. They cannot have a corresponding dominant allele on their Y chromosome, so they develop the condition.

Females whoa re heterozygous for the haemophilia coding gene are carriers. They don't suffer from the disorder, but they may pass on the allele.

Another example of Sex Linkage

Calico cats

Codominance

Codominance occurs when 2 different alleles occur for a gene, both of which equally dominant.

As a result, both alleles of the gene are expressed in the phenotype of the organism if present.

What type of cell division occurs in sexual reproduction?

Meiosis

Genetic Variation in Meiosis

• Crossing over in prophase I

• Independent assortment at metaphase I and II

• Random fertilisation of gametes