Genetics & Inheritance

Genotype

Genetic constitution of an organism determined by the combo of alleles that an organism inherits from parents

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Phenotype

The expression of genotype and its interaction with the environment

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Gene

Sequence of bases on DNA that codes for one polypeptide

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Alleles

Alternative versions of the same gene

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Locus

The position of a gene on a particular DNA molecule

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Homozygous

Alleles for a characteristic that are the same

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Heterozygous

Alleles for a characteristic that are different

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Dominant Allele

Always expressed in phenotype if it's present in genotype

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Recessive allele

Only expressed in phenotype if dominant allele is absent

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Pure Breeding

Crossing with another organism the same characteristic, all of the offspring also have that characteristic. Pure breeding= Homozygous

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Features needed when investigating inheritance of specific genes/alleles

-The organism must show some discontinuous variation=Allows to clearly distiniguish the characteristics tested -Must reproduce sexually= allows to see the effect of recombining alleles -It is pure breeding=Alleles of parents are known

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Monohybrid Crosses

If inheritance of one pair of alleles is studied.

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Testcross/Backcross

Determining genotype of an unknown organism. Cross the unknown with a recessive phenotype- if any resulting offspring are the homozygous recessive phenotype the unknown genoptype=heterozygous if ALL offspring dominant phenotype unknown=homozygous dom

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Co-dominance

When a gene has two alleles are neither can dominate, if both alleles are present in genotype then both expressed in phenotype, a third phenotype arises when there is a heterozygous genotype

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Multiple Alleles

Some genes have more than 2 allelic form. Each individual can only have 2 alleles for each gene. Blood groups- Alleles A&B= co-dominant Allele O= Recessive to both A&B

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Sex Determination

Gender is determined by combination of chromosomes in a Zygote. X=larger than Y, Human females=**(22**) Human males=XY(22XY),it is the male gamete that determines the sex of offspring(reversed in e.g.birds)

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Sex Linkage

In males two sex chromosomes are non-homologous= x&y differ in size and gene content. Many genes on X will not have equivalent allele on Y chromosomes. A recessive allele on an X chromosome-much more chance being expressed in the phenotype of a male

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Sex-linked diseases caused by recessive X chromosome

-Haemophilia -Colour blindness -Muscular Dystrophy

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Pedigrees

Evidence FOR sex-linkage in pedigree diagrams=look for greater no.of affected males than females/no affected females. Evidence AGAINST=F w/recessive phenotype has a son/father w/dominant phenotype-not possible if gene on X sons inherit x w/recessive

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Dihybrid Inheritance

Shows how 2 pairs of contrasting characteristics are inherited. E.g. Tall purple plant + Dwarf white

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Epistasis

Arises when allele of one gene affects the expression of another in the phenotype

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Normal ration expected on a heterozygous dihybrid genetic cross

9:3:3:1

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Autosomal linkage

non-sex chromosomes->autosomes. Genes on the same chromosomes are said to be linked- will stay together during independent assortment in M1, their alleles will be passed on to offspring together- so fewer types of gametes will form.

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Crossing over

Exchange of genes between chromatids of homologous chromosomes during meiosis1

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Genetics & Inheritance

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