Inheritance 1

Mendel’s first law of segregation

during formation of gametes, the paired alleles segregate randomly so that each gamete receives one or the other with equal likelihood.

Mendel’s second law of independent assortment

The segregation of one pair of alleles is independent of the segregation of the other

Why are females described as homogametic?

• The female produces one type of gamete (all egg cells carry an X chromosome)

Why are males described as heterogametic?

• The male produces two types of gametes

• half sperm cell carry an X chromosome while the other half carries a Y chromosome

Sex-linkage

• the carrying of genes on the sex chromosome

X-linked genes or sex-linked genes

genes located on the sex chromosome, mainly the X chromosome

Why does sex-linked inheritance mainly affect males

• Males are hemizygous as they have possess only a single X chromosome and hence have only one sex-linked gene locus (instead of 2 as seen in females)

• whatever allele present on X chromosome of males ,regardless of whether it is dominant or recessive, will be directly expressed in the phenotype (recessive allele will be expressed due to no absence of a dominant allele to mask it)

What is a reciprocal cross? what is the purpose and expected positive result?

What: pair of crosses in which the traits of the two parents are reversed

Purpose: A reciprocal cross can be conducted to discern if a trait is carried on a sex chromosome(X chromosome) or an autosomal chromosome

Positive result: yield non-identical results in contrast to reciprocal crosses involving autosomal traits which yield identical results

characteristics of x-linked recessive inheritance

• most affected individuals are male

• affected mothers produce affected sons with 100% chance

• affected females are the result of mating between affected fathers and affected/carrier mothers

characteristics of x-linked dominant inheritance

• affected mother will produce affected sons with only 50% chance

• affected females are the offspring of affected mothers or father

• All daughters but none of the sons are affected, assuming mother is normal

Characteristics of dominant inheritance

• unaffected parents does not have affected children

• trait does not skip generations (affected individuals will always give rise to affected offspring)

• when affected individuals meets unaffected individual. 50% offspring should be affected

Characteristics of recessive inheritance

• both parents are affected, all children are affected

• unaffected parents can produce affected individuals

• trait often skips generations

Linked genes

• genes that control different characters and situated on the same chromosome at different loci (transmitted as a single unit during segregation of homologous chromosomes)

• fail to produce the expected 9:3:3:1 or 1:1:1:1 ratio in independent assortment in dihybrid inheritace

What does complete linkage mean for the resulting phenotype?

• no crossing over occurs betwen linked genes and they are transmitted as a unit during anaphase 1

• all linked genes will pass into the same gamete, resulting in only parental gametes

• will show a monohybrid offspring phenotypic ratio (3:1 or 1:1 which deviates from expected Mendelian ratios 9:3:3:1 or 1:1:1:1)

• results in a parental phenotypes and no recombinant phenotype produced

Why does complete linkage occur?

• two pairs of genes must be located very close together on the same chromosome and are closely linked together

What does incomplete linkage mean for the resulting phenotype?

• crossing over occurs between the linked genes in some cells, forming recombinant chromatids in

• leading to a new combination of alleles in the gametes

• and hence new combinations of alleles in offspring, called recombinants

• offpsring show a majority of parental allele combinations and hence parental phenotypes, and a minority of recombinant allele combinations and hence recombinant phenotypes (deviates from expected Mendelian ratios 9:3:3:1 or 1:1:1:1)

Why does incomplete linkage occur?

• as distance between linked genes on the same chromosome increases

• the probability of crossing over happening between the linked genes increases

How should we detect if gene are unlinked, completely linked or incompletely linked?

Do a test cross between a double heterozygous and double homozygous recessive

• unlinked(diff chromosomes): phenotypic ratio of 1:1:1:1 (4 phenotypes)

• completely linked: phenotypic ratio of 1:1 (2 parental phenotypes)

• incompletely linked: no fixed ratio but larger percentage of parental phenotype and smaller percentage of recombinant phenotype (2 parental phenotypes + 2 recombinant)

Coupling arrangement of linked genes

two dominant allels are on one chromosome and the two recessive are on the homologous partner

Replusion arrangement of linked genes

dominant allele is linked to a recessive allele on one chromosome

What does the Crossover Value (COV) or Recombinant Frequency (RF) represent?

the value can be used to represent the relative distance between genes in arbitrary units.

This is because the chances of crossing over between the linked genes are proportional to the distance between them.

How to calculate the Crossover Value (COV) or Recombinant Frequency (RF)?

note: By convention, a COV of 1% represents a relative distance of 1 centimorgan(cM).

What are the effects of the genotype on the phenotype?

1. gene mutation: change in nucleotide sequence of DNA results in formation of a new allele

2. Chromosome mutation: change in structure of chromosome or no. of chromosomes

3. Meiosis: crossing over of non-sister chromatids during phrophase 1 and independent assortment of homologous chromosomes during metaphase 1(independently align along the equator) —> genetic variation with new cominations of existing alleles

4. Sexual reproduction: random fusion of gametes —> genetic variation with new cominations of existing alleles

What are the effects of envrionment on phenotype?

1. Effect of diet on animals (eg queen bee larvae feed on royal jelly)

2. Effect of elevation on plants (eg yarrow plants grow tall at lowest and highest elevation)

3. Effect of soil acidity on plants (eg hydrangea flowers are blue in acidic soils and pink in pH 6.5 or higher)

Effect of diet on bees

• all larvae are initiallly fed with royal jelly

• worker bees are switched to a diet containing honey and pollen

• larvae destined to be the queen bees are fed with royal jelly containing high content of protein which stimulates the formation and maturation of the female reproductive system