GH lecture 4 - Mendelian genetics 2

The law of segregation:

Alleles segregate independently during gamete formation

What is Mendel's first law?

The law of independent assortment:

alleles for separate traits are passed independently of one another

What is Mendel's second law?

crossing 2 individuals who are heterozygous for 2 different traits

What is a dihybrid corss?

9:3:3:1

(double dominant):(dominant for trait A recessive for trait b):(dominant for trait B recessive for trait a):(double recessive

What is the phenotypic ratio expected for a dihybrid cross of the F2 generation?

9 (AABB, AABb, AAbb, AaBB, AaBb, AaBB, aaBB, aaBb, aabb)

How many genotypes can arise from a dihybrid F1 cross?

They can, but won't necessarily, be moved to another chromosome during crossing over (splicing) in metaphase 1

How can genes on the same chromosome be passed on to offspring independently?

The closer the 2 loci are on the chromosome, the less chance there is for crossing over to occur between them. Therefore, the closer 2 genes are together, the more likely they will be passed on together

When looking at genes on a shared chromosome, what determines how likely they will inherited together (linkage)?

The distance between genes can be calculated by the percentage of gametes in which the genes are separate. For every 1% of the gametes in which the genes have separated, there is said to be 1 Map Unit (m.u.) aka 1 centimorgan (cM) of distance between the genes. 1 centimorgan corresponds to approximately 1 million base pairs.

How is distance between genes measured?

Genes are linked if they are recombinant (split) in less than 50% of offspring

What defines a gene linkage?

A hybrid individual for all genes is crossed with an individual which is recessive for all genes. The ratios of resulting genotypes gives insight into whether recombination has occurred. Eg: If only 10% of offspring had recombinant genes and 90% shared their parental genotype, the genes are 10cM apart

How is a test cross used to determine linkage?

A-B-C

If genes B and C are 10 m.u. apart, genes A and B are 40 m.u. apart, and A and C are 50 m.u. apart: propose the order in which they may sit on the chromosome

Gene A is said to be epistatic over Z if a certain genotype for gene A masks the effects of the genotype of gene Z Eg: Coat colour in labradors is normal either B(black) or b(brown), but if the dog is homozygous recessive for a gene ee at another locus, this creates a yellow coat, regardless of the B/b genotype ie:

eEBb = Black

EEbb = Brown

Eebb = Brown

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.

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eeBB = yellow

eebb = yellow

eeBb = yellow

Explain epistasis

Individuals with this phenotype have the recessive hh genotype which overrides their A/B/i genotype and makes them blood type O

What is the Bombay phenotype?

pleiotropy is when a gene has an effect on more than 1 visible character (for example seen in genes responsible for pigmentation)

What is pleiotropy?

The homozygous recessive genotype may be lethal

In a scenario where you are trying to produce a recessive phenotype and you continue to fail no matter how many heterozygotes you cross, what might be the cause?

-mutation at the coding region (exon), resulting in an altered protein with varying functionality (CYstic fibrosis)

-mutatin at the non-coding region (intron), inducing incorrect splicing (osteogenesis imperfecta)

-muation at a regulatory sequence, causing failure or incorrect expression (b thalassemia - b-globin locus control region defect)

What are 3 ways in which an allele may be mutated

wild types

Are wild types or mutations generally dominant?