Inheritance

Gene

The sequence of DNA that occupies a specific site on a chromosome and codes for a polypeptide.

Genotype

All the alleles that an individual contains

Phenotype

An individuals appearance and characteristics

Monohybrid inheritance

The inheritance of a single gene

Why did Gregor Mendel experiment on garden peas to investigate inheritance

• They are easy to grow 

• Their flowers can self-fertilise and cross fertilise 

• They make flowers and fruit in the same year 

• They make a large number of seeds from each cross. This means that when the phenotypes of the next generation are counted, their numbers make them statistically meaningful. 

What do genetic diagrams show

• The generations 

• The genotypes of parents and offspring  

• The phenotypes of parents and offspring  

• The alleles present in the gametes 

• The symbols for the alleles are defined 

F1

first filial generation

F2

second filial generation

What does the test cross show

if a dominant characteristic is determined by one or two dominant alleles

Co-dominance

both alleles in a heterozygote are expressed individually

Incomplete dominance

the phenotype of the heterozygote is intermediate between the two parental phenotypes rather than their both being expressed

Independent assortment

The maternal and paternal chromosomes, and therefore the alleles they contain, mix with each other in any combination in the gametes.

Dihybrid inheritance

the simultaneous inheritance of two unlinked genes

Second law of inheritance

Either one of a pair of contrasted characters may combine with either to another pair AKA each member of a pair of alleles may combine randomly with either of another pair on a different chromosome

Linked alleles

The alleles of two genes that are on the same chromosome cannot segregate independently so cannot move to opposite ends of the cell at meiosis. They are on the same physical structure so must move together.

Factors affecting whether an individual animal is male or female

• Temperature

• Sequential hermaphroditism

• The absence of females

• Ploidy level

• Chromosome structure

How does temperature affect whether an individual animal is male or female

lizard eggs hatch as male when the temp is above 32 degrees Celsius, sea turtle eggs hatch as females if laid in full sun but males if laid in the shade

How does sequential hermaphroditism affect whether an individual animal is male or female

the common slipper limpet (mollusc) makes stacks of individuals with those at the top being male but as more males join the top, those below them become females

How does the absence of females affect whether an individual animal is male or female

the male sewage worm, Capitella, can become hermaphrodite and fertilise itself if females are not available. Clownfish live in hierarchies so when the dominant female dies the dominant male will change sex to replace her

How does ploidy level affect whether an individual animal is male or female

bee eggs that aren’t fertilised are haploid and develop as males but fertilised eggs are diploid and develop as females

How does chromosome structure affect whether an individual animal is male or female

in mammals, females are XX and males are XY. In birds, females are ZW but males are ZZ

Karyotype

the arrangement of homologous pairs in decreasing order

Autosomes

homologous pairs with identical genes but different alleles

Heterosomes

Sex chromosomes which are different sizes

Pseudoautosmal regions

the 2 regions on the human X and Y chromosomes that are homologous and can pair with each other at meiosis

Which sex is homogametic

females as their secondary oocytes all contain an X chromosome

Which sex is heterogametic

males as half their sperm contains an X chromosome and the other half contains a Y chromosome

Sex-linked condition

a condition that is far more common in one sex compared with the other

How is DMD a sex-linked condition

A male won’t pass the alleles on his X chromosome to his sons since they receive the Y chromosome but his daughters all receive an X chromosome from him. DMD is an X-linked recessive allele of the dystrophin gene.

What can a pedigree diagram indicate in terms of conditions

whether the condition is sex-linked and whether it’s the result of a dominant or recessive allele