Genetics: Inheritance

12 Biology Final Review

Gregor Mendel

-determined that parents pass discrete heritable factors on their offspring, which retain individuality generation after generation.

-each parent passes down a factor, one may be dominant over the alternative form.

Mendel’s Law of Segregation

-Each individual has 2 factors (genes) for each trait that separate during the formation of gametes.

-Each gamete contains only one factor from each pair of factors

chromosomes

-come in homologous pairs which have genes controlling the same traits. genes are located at the same point/locus on each member of the pair 

-genes are sections of chromosomes

alleles

alternative forms of a gene for a trait

Dominant Alleles

-represented by the capital letter

-a certain traits will result if the individual has at least 1 dominant allele

Recessive Alleles

-represented by the lowercase letter

-for a recessive trait to result the individual must have 2 copies of the recessive allele

Genotype

-genetic composition of an individual with regard to a specific trait

-may either be homozygous dominant, heterozygous, homozygous recessive Ho

Homozygous Dominant

2 copies of the dominant allele

Heterozygous

1 copy go the dominant allele and 1 of the recessive

Homozygous Recessive

-2 copies of the recessive allele

Phenotype

-physical appearance of the individual with regard to a trait

-homozygous dominant and heterozygous individuals will have the same phenotype, whereas the homozygous recessive individual will have a different phenotype

Gamete Formation

-during meiosis, homologous chromosomes separate so there is only 1 member of each pair in a gamete

-there is only 1 allele for each trait in each gamete

-no 2 letters in a gamete can be the same letter of the alphabet:

o if the genotype is Ww, then gametes from this individual will contain either W or w

o whereas if the genotype is WwLl (2 traits) gametes can be the following: WL, Wl, wL, wl

Punnet Squares

easy way to figure out all possible combinations of eggs and sperm

Genetic Ratios

-In a punnet square we had the following offspring: WW, Ww, Ww, ww

• the genotypic ratio is 1 WW: 2 Ww: 1 ww

Genetic Probability (product rule)

-another way to phrase the phenotypic ratio is in terms of probability

- the chance of 2 or more independent events occurring together is the product of their chance of occurring separately

Genetic Probability (sum rule)

the chance of an event that occurs in more than one way is the sum of the individual chances

test cross

-since homozygous dominant individuals are phenotypically the same as heterozygous individuals (both appear the same) test crosses are utilised to determine the likely genotype of an individual :

Law of Independent Assortment

-alleles of 2+ different genes are sorted independently of other genes.

-whichever allele is received by a gamete for 1 gene doesn’t affect which allele for a different trait is received

-gamete can receive any possible combo of alleles

Dihybrid Cross

Two-trait Test Cross

-cross an individual with the dominant phenotype for each trait with an individual with the recessive phenotype of both traits

Polygenic Inheritance (Bell Curve)

- controlled by 2 or more sets of alleles, each dominant allele codes for a product and effects are additive

-result is a continuous range of phenotypes, where the distribution resembles a bell curve

Polygenic Inheritance (combo)

-parakeet feathers are controlled by 2 genes, B (blue) and Y (yellow)

-green parakeets have at least 1 dominant allele for each gene, and white parakeets have only the recessive alleles

Environmental Influences

-enviornment can influence gene expression, and therefore phenotype.

-human twin studies show that polygenic traits are most influenced. if they share a common trait despite being raised in different households, it is likely genetic

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Incomplete Dominance

-heterozygous individuals have a phenotype intermediate to the 2 homozygous individuals.

• ex. curly hair woman and straight hair man produce wavy hair kids
  • snapdragons are either R/red or r/white, so a Rr flower is pink

co-dominance

-occurs when both alleles are equally expressed

• ex. blood type AB represents A and B equally
  • rhododendrons exhibit both white and pink petals

multiple allele inheritance

-gene exists in several allelic forms, but each person still has only 2 of the possible alleles

• ex. ABO blood types:
  • Ia= A antigens on red blood cells
  • Ib= B antigens on red blood cells
  • I= neither A or B antigens
  • phenotype: A → genotype: IaIa or lal
  • phenotype: B → genotype: lblb or lbl
  • phenotype: AB → genotype: lalb
  • phenotype: O → genotype: II

Paternity Testing

- ABO blood groups often used

• can disprove paternity, but not prove it

-Rh factor:

• another antigen on the RBC, where Rh + has the antigen and Rh - does not
• multiple alleles for Rh - but all are recessive to Rh positive

sex chromosomes

-22 pairs of autosomes, 1 pair of sex chromosomes (therefore 23 total)

- XX: female sex chromosomes, XY: male sex chromosomes

sex linked traits

-traits controlled by genes in the sex chromosomes

- X chrom. has many genes, whereas the Y does not

Carriers

-when a female is heterozygous for a sex linked trait, therefore able to pass onto her children

X linked traits

-as males only have one X chromosome, recessive traits are more common in men

- ex. colourblindness is x-linked

• the X chromosome has genes for normal colour vision, so XB= normal vision and Xb= colourblindness

Linked Genes

-a chromosome has several genes, and the sequence of those genes is fixed bc each allele has a specific locus.

-all genes on a single chromosome form a linkage group. when linkage is complete, a dihybrid produces only 2 types of gametes

-any time traits are inherited together, a linkage group is suspected. or, if very few recombined phenotypes appear in offspring, linkage is also suspected