Genetics

Definitions

genetics

  • study of heredity
  • way in which characteristics pass
  • from parents to offspring

genes

  • short sections on chromosome
  • code for production
  • of specific polypeptide

heterozygous

  • alleles of same gene
  • DIFFERENT

homozygous

  • alleles of same gene
  • IDENTICAL

alleles

  • particular form of gene
  • located at the same locus
  • on homologous chromosomes
  • in diploid organisms

genotype

  • 2 alleles for a particular characteristic

phenotype

  • outward appearance of individual
  • in terms of particular characteristic

gene locus

  • position of gene
  • on chromosome

History of Genetics
Gregor Mendel

work on breeding peas

  • formed basis of genetics

Monohybrid Inheritance
inheritance of alleles @ 1 gene locus

in pea plants -

  • many characteristics varied in discrete way
  • from plant to plant
    • height
    • pea shape + size
    • flower colour

M investigated
pea height

  • normal
  • tall

why peas?

  • easy to cultivate
  • plant self-pollinating
  • easily observed differences
  • hybrids = completely fertile

Mendel's Experiment

1.allowed tall plants to interbreed

  • ensure produce ALL tall
  • eg pure breeding (homozygous)
    **did same w/ small plants

2. crossbred tall + small (parental generation)

  • ALL offspring = TALL
  • (F1 generation = offspring of cross between 2 parental genotypes)

3. two F1 plants crossed

  • offspring contain MIX SHORT + TALL
    (F2 generation - offspring resulting from cross 2 F1 individuals)
  • tall:short - 3:1 ratio

Conclusions

  • only 1 of alleles in parental genotype pass to gamete


    • during meiosis
    • 1 homologous chromosome from each pair
    • into gamete

  • totally random which allele enter gamete

Law of Segregation
MENDEL'S FIRST LAW

  • 2 alleles of each gene
  • separate during meiosis
  • with only 1 entering each gamete

D: inheritance of the alleles

  • of a single gene
  • at 1 gene locus

dominant

  • in heterozygous condition
  • DA OVERRIDE non-dominant allele

recessive

  • will be dominated by DA
  • only show itself in phenotype if 2 R

Test/Back Cross

to determine GENOTYPE

homozygous dominant + heterozygous = SAME PHENOTYPE
= DIFFERENT GENOTYPE

3:1 RATIO
F2 generation

1:1 RATIO
F1 generation

parental genotype

  • homozygous dominant
  • homozygous recessive

parental genotypes

  • heterozygous (Tt)
  • homozygous recessive (tt)

F1 generation genotype

  • all heterozygous

method

  • cross unknown individual
  • w/ individual known genotype
    • homozygous recessive

conclusions

  • if ALL offspring is tall
    • unknown = TT
  • if 50% tall + 50% short
    • unknown = Tt

Co-dominance

Multiple Alleles

Sex Determination

Dihybrid Inheritance
inheritance of alleles @ 2 gene loci

Polygenic Inheritance
(gene interaction)

Epistasis
(gene interaction)

2 alleles @ same gene locus

  • neither dominant/masking effect

both co-dominant alleles make
contributions to heterozygous phenotype

1:2:2 RATIO

  • phenotype ratio
  • 1M:2MN:1NN

more than 2 alleles of particular gene

  • eg ABO blood groups

A and B
dominant to O

A co-dominant
with B

Lethal Alleles

1:1:1:1 RATIO

  • heterozygous group A = IAIO
  • heterozygous group B = IBIO

eg house mouse
coat colour

EXPECTED RATIO
3 (yellow):1 (agouti)

OBSERVED RATIO
2 (yellow):1 (agouti)

homozygous dominant yellow (YY) always dies

  • 2 alleles coding for yellow
  • lethal combination
  • fail to develop properly
  • die as embryo

result in:

  • spontaneous abortions embryos/foetus
    OR
  • death affected individuals in adulthood (with no prior warning) - eg Huntington's

require:

  • both recessive
    OR
  • single dominant autosomal
    (eg )Huntington's

Y chromosome

  • smaller than X
  • ONLY carries gene for maleness
  • codes for protein to switch on direct development of testes in embryo

autosomes (not sex chromosome)

  • homologous chromosomes
  • identical in appearance

heterosomes

  • homologous chromosomes
  • not identical in appearance

female = homogametic

  • XX
  • sex chromosomes identical

male = heterogametic

  • XY
  • sex chromosomes non-identical

50:50 RATIO

Sex Linkage

sons NEVER inherit father's X chro

  • sons NEVER inherit father's sex linked character.

daughters ALWAYS inherit father's X chr

  • ∴ 50% chance passing sex linked to offspring

conditions caused by

  • single RECESSIVE ALLELE
  • on X chromosome

examples

  • red-green colour blindness
  • haemophilia
    (abnormally long time clot blood)
  • Duchenne muscular dystrophy

recessive in females

  • (not "recessive in males" = only recessive to dominant & males only have 1 allele)

why more common
in males?

males only need 1 recessive allele

  • only need 1 from mum
    (carrier/have condition)

females need 2 recessive alleles

  • 1 from each parent
  • dad must have condition
  • mum = carrier/have condition

carrier

  • has recessive allele
  • but is not affected
  • as also carries D

WORKING OUT PROBABILITY

  • "colour blind" son - P (son) x P (colour blind)
  • "son that is colour blind"

Mutations

causes 2 or more
alternative alleles

most often mutated
allele = recessive

simultaneous inheritance
of 2 characteristics

Law of Independent Assortment
MENDEL'S SECOND LAW

  • during the formation of gametes
  • segregation of alleles of 1 gene
  • INDEPENDENT
  • to segregation of alleles of other gene

9:3:3:1 RATIO

  • homozygous dominant - TTRR
  • homozygous recessive - ttrr

1:1:1:1 RATIO

  • heterozygous - TtRr
  • homozygous recessive - ttrr

independent inheritance

  • if genes on different chromosomes

is complete range of
intermediate phenotypes
eg. height

D: gene interaction when 1 gene
masks/modifies action of other gene

results in unusual
genetic ratio

examples


2 loci INTERACT

  • create NEW PHENOTYPE
  • comb type = chickens
    OR
  • flower colour = sweet pea

2 genes DUPLICATE work of each other

  • wheat kernel colour
  • 15:1 ratio

allele @ 1 locus MASK allele @ other locus

  • squash colour

explained during metaphase I (meiosis)

  • homologous pairs of chro (bivalents)
  • arrange themselves independently
  • on equator of spindle

CONTINUOUS
VARIATION
(bell-shaped curve=normal distribution)

CUMULATIVE effect of
more than 1 gene

D: refers to characteristic

  • controlled by many genes
  • with individual slight effect on phenotype

features:

characters controlled
by no. of genes

genes individual small
effects

phenotype subject
to environmental variation

  • EFFECT OF ENVIRONMENT
  • height = diet etc
  • genes switched on = epigenetics

characters show continuous
range of variation