Inheritance & Epigenetics

Gene

The length of DNA on a chromosome that codes for the production of 1 or more polypeptide chains and functional RNA

Chromosome

a structure consisting of a long, coiled molecule of DNA and its associated proteins, by which genetic information is passed from generation to generation

Homologous pair

a chromosome pair, one paternal and one maternal, with the same gene loci

Chromatid

one strand of a replicated chromosome

Centromere

the structure on a chromosome that links the two sister chromatids

Genotype

the genetic constitution of an organism

Phenotype

the expression of an organism’s genetic constitution, combined with its interaction with the environment

Monohybrid inheritance

where one phenotype characteristic is controlled by 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. 

Mendel’s first law of inheritance

• Law of segregation

• Alleles separate randomly into gametes

• Each parent passes one allele to their offspring

Mendel’s second law of inheritance

• Law of independent assortment

• The alleles of genes assort independently of other genes during gamete formation

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

used to determine whether an individual with a dominant trait is heterozygous or homozygous dominant

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

A chromosome that is not an X or Y chromosome

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

Addition gene mutation

a base is added to the base sequence

Duplication gene mutation

the same base is incorporated twice into the base sequence

Subtraction gene mutation

a base is deleted from the base sequence

Substitution gene mutation

a different base is incorporated into the base sequence

Inversion gene mutation

adjacent bases on the same DNA strand exchange position

What is an example of a substitution gene mutation

Sickle cell anaemia

Chromosome mutation

changes in the structure or number of chromosomes in cells

Down’s syndrome

a secondary oocyte has 2 copies of chromosome 21 that fuses with a normal sperm produces a viable embryo with cells containing 3 copies of chromosome 21. This is called trisomy 21 and produces Down’s syndrome

Translocation Down’s

during meiosis in a gamete that produced them, a fragment or chromosome 21 attaches itself to chromosome 14. When that abnormal gamete fuses with a normal one, it produces an embryo with 2 normal copies of chromosome 21 and an additional 1 attached to chromosome 14

Euploid

cells with complete sets of chromosomes

Aneuploid

cells with a small number of extra chromosomes or a small number too few

Polyploid

cells with several sets of chromosomes

2 reasons why polyploidy is more common in plants than animals

plants can reproduce asexually and are hermaphrodite so don’t use chromosomes to determine their sex

Carcinogenic

an agent that causes cancer

Tumour suprressor genes

genes that regulate mitosis and prevent cells dividing too quickly

Proto-oncogene

codes for a protein that contributes to cell division

Epigenetics

the control of gene expression by modifying DNA or histones, but not by affecting the DNA nucleotide sequence.

DNA methylation

a methyl/hydroxymethyl group is added to cytosine. Methylated cytosine pairs with guanine at transcription. But if regions of DNA are heavily methylated, they are less likely to be transcribed

Histone modification after translation

an acetyl group is attached to the amino acid lysine/a methyl group to lysine/arginine or a phosphate group to serine/threonine. The interaction of of histones with DNA and the arrangement of the nucleosomes is changed. Coiling of DNA around histones relaxes which increases gene transcription

Stem cell

an undifferentiated cell that contains the individuals entire DNA so expresses all genes, it’s found in bone marrow or as a zygote

Genome

an individuals entire DNA composition

Somatic cell

a general body cell

IPS

induced pluripotent stem cells, used to grow tissues

Pluripotent cell

cells that are undifferentiated that can divide in their undifferentiated state

Germline cells

Gametes (sperm and egg)