5.4.2

Question 1

Gregor Mendel heredity (Year 10 recap)

Answer 1

• Austrian monk who experimented with the selective breeding of peas
• Found that inheritance does not occur according to the blending model in which traits from each parents are averaged together
• Found instead that most characteristics are passed directly from one generation to another, but some skipped several generations or at a ratio of 1:1 or 1:3 (one out of 3 plants were green or yellow)
• Came up with the idea of recessive or dominant alleles

Question 2

Mendel’s pea experiment

Answer 2

• Crossbred two purebred peas (one green, one yellow)
• The offspring produced were all yellow. This was the F1 generation
  In the F2 generation produced by interbreeding the F1 members, one offspring out of 4 was green
• Concluded that yellow is dominant, the green was a recessive gene.

Question 3

Heterozygous vs homozygous

Answer 3

• If the alleles are both the same, it is homozygous. If both alleles are dominant, it is homozygous dominant. If both are recessive, it is homozygous recessive.
• If the alleles for the same gene are different, it is heterozygous.

Question 4

Mendel’s law

Answer 4

• Genes in homologous pairs can be for an alternative form of the same trait. E.g. the gene for hair colour might be blond or brown, the gene for hair style might be curly or straight.
• Polygenic inheritance: a trait whose expression is influenced by multiple alleles, not just one. E.g. eye colour is not determined by the dominance of a single allele, e.g. B over b, but influenced by many other factors.

Question 5

Mendelian ratios

Answer 5

• Ratio of the likelihood of particular traits being expressed in his F1 and F2 peas based on the traits of the parent plants, P
• Only applies to traits that are controlled by a single gene, called monohybrid crosses

Question 6

Dihybrid cross

WILL NOT BE IN THE TEST

Answer 6

• When 2 traits on different chromosomes are observed over generations
• Ratio for F2 is 9:3:3:1 (Dominant, mixed, mixed, recessive)

Question 7

Linked genes

WILL NOT BE IN THE TEST

Answer 7

• Some genes do not follow Mendelian ratios and appears to break the Law of Independent Assortment
• These are likely to be found on the same chromosome linked genes
• Linkage group: a set of genes at different loci on the same gene that except for crossing over, tend to be inherited together

Question 8

Morgan experiment

Answer 8

White eyes were expressed only with male flies.
The gene for eye colour located on the X sex chromosomes (only male XY)

The discover of genes located on allosomes but not directly related to sex is sex linkage.

Question 9

Allosomes

Answer 9

sex chromosomes

Question 10

Sex linkage in humans eg

Answer 10

X: Red-green colour blindness and haemophilia
Y: facial hair
Sex linkage is a type of gene linkage - in which there is different chance of phenotype expression in female v. male.

Question 11

Non-Mendellian Rations

Answer 11

Incomplete dominance:
* Dominant allele is not showing its complete dominance. Equal dominance (?) Present in heterozygous offspring.
* Ratio 1:2:1 (homozygous dominant:heterozygous mixed:homozygous recessive)
e.g. snapdragon plants can express pink flowers.

Codominance:
* Both alleles express themselves to produce a “pathwork” phenotype.

Polygenic Inheritance:
* For continuous traits, in which a range of expressions controlled by several genes or alleles. A trait controlled by two or more genes (polygenes) contributing to the phenotype.
* Previously we have been looking at traits that are easily deteced as being discontinuous (present or not) that are the expression of a single allele.
* Height, eye colour, skin tone.

Question 12

Multiple allele trait

Answer 12

Normally a gene has two alleles - however, in this case one gene has multiple alleles.
E.g. blood type in which the allels are codominant (expressed equally)

Question 13

Codominance

Answer 13

Two alleles are expressed equally in the heterozygous phenotype.