Mendel - Genetic Inheritance Flashcards
Genetics
is the study of heredity in general and of genes in particular
e.g eye colour, hair colour
Heredity
the passing of traits from parent to offspring (INHERITANCE)
Phenotype
individual’s observable traits (such as height, eye colour, etc)
Genotype
the set of genes in our DNA which are responsible for a particular trait
traits (phenotype) are influenced by MULTIPLE genes and the environment
patterns of inheritance are complex and unpredictable
A small number of traits are influenced primarily by a single gene
They have predictable patterns of inheritance (Mendel’s laws of heredity)
Mendel
documented the mechanism of inheritance genetic in sexually reproducing organisms
In order to study inheritance, Mendel chose to use peas:
> they are easy to cultivate
Available in many varieties
Produce large numbers of offspring
Strict control over the mating/breeding
Only study traits can be OBSERVED / obvious
Pea colour – green vs yellow, with no other variants
True breeding
When the plants self-pollinate, all their offspring are of the same variety
Hybridisation
Mating, or crossing, of two varieties
Monohybrid cross
A cross between two parents that breed true for different versions of a single trait
P generation
True breeding parents
F1 generation
(first filial) Hybrid offspring of the P generation
F2 generation
(second filial) Offspring from the self-fertilisation of the F1 hybrids
Mendel cross-pollinated two true-breeding varieties (P generations, parents), studying each trait one at a time
Yellow and green pea colour
All the F1 hybrids resembled only one of their two parents (yellow peas)
Yellow pea was dominant
Mendel allowed the F1 plants to self-fertilise and observed the F2 (2nd generation offspring)
the trait that disappeared in F1, returned
75% of the offspring had yellow peas, 25% had green peas
inheritance of traits is governed by hereditary factors: which we now call…
genes
E.g. Stem height, pea colour
These factors come in alternative versions and organisms have pairs of factor versions; called…
alleles
tall/dwarf, yellow/green
An organism must inherit two copies, or alleles, of each gene
1 from its mother + 1 from its father
He theorised, true-breeding parental strains, possessed a pair of identical alleles
- Homozygous
- Heterozygous
Homozygous
An organism possessing two identical alleles for that trait (gene)
The F1 hybrid plants, on the other hand, had received two dissimilar alleles
Heterozygous
An organism possessing two different alleles for that trait (gene)
Mendel’s second postulate – Dominance/Recessiveness
(2nd law)
The dominant allele of the gene (denoted by a capital letter, e.g. T=TALL stem)
The recessive allele of the gene (denoted by a small letter, e.g. t=dwarf stem)
The appearance, of organism depends on which versions of each allele it inherits
Phenotype
What type of chart do we use?
Punnet chart
In Heterozygous genotype (Tt), the dominant one dictates the organism’s phenotype Tt The stem will be tall
heterozygous dominance
T ‘wins’ - more powerful
tt
SHORT
because two homozygous recessive alleles are present and therefore showing the phenotype
Mendel’s third postulate – Independent Segregation
two copies (alleles) of each gene present in the parent separate so that each gamete receives only one allele for each trait
During fertilisation, a sperm cell carrying one or other allele fuses with an egg cell carrying one of the alleles to produce:
a zygote with two alleles
The two alleles for each trait separate (or segregate) during gamete formation and then
> unite at random—one from each parent—at fertilisation
- In F1, half the gametes will get a yellow-pea allele and half will get a green-pea allele
- Four combinations are possible in F2, explaining the 3:1 ratio
Mendel’s Postulates - Albinism
The major form of albinism, Type II albinism follows the Mendel’s Law of Segregation (1 gene involved)
Disorders that results in little or no production of the pigment melanin
(colour of the skin, hair and eyes)
What is the role of the dominant allele of the gene (A)
encodes an enzyme involved in making melanin, the recessive one is non/low active (a)
Alleles for Different Traits Segregate Independently
Mendel’s law of independent assortment states that, during gamete formation, different pairs of alleles segregate independently of one another
Alleles that segregate independently are packaged into gametes in all possible combinations
4 combinations of gametes
These gametes are allowed to combine at random to produce the F2 generation,
Double dominant phenotype, two single dominants and the double recessive phenotype
What ratio is the double dominant phenotype in?
9:3:3:1
