1800s blending theory of inheritance
offspring have characteristics of both parents, so offspring must be a blend of the parents’ generation
how was the blending theory disproved?
would gradually lose diversity due to this, blue-eyed children to brown-eyed parents disproved this
how did mendel employ the scientific method?
careful experimentation with controlled crosses and qualitative analysis
mendel’s steps to controlled crosses
- cutting off the anther prevents the plant from self-pollinating
- brushing the pollen of the purple plant on the stigma of the white plant is cross-pollination
- the new seeds that grow from the resulting seeds reveal which trait was dominant and which was recessive
why were pea plants chosen?
easy to grow, low generation time, distinct characteristics, easy to mate
mendel’s conclusions
- variation in traits is due to different alleles
- alleles segregate randomly into gametes
- organisms inherit 2 alleles for each trait
- appearance of heterozygotes is determined by dominant traits (recessive alleles masked for many generations)
principle of segregation
two alleles of a gene segregate from each other during gamete formation
principle of independent assortment
during the segregation of alleles into gametes, alleles of different pairs assort independently
mendel’s principles
segregation and independent assortment
what causes different alleles?
due to mutations or SNPs, don’t have to occur at the same place, only on the same gene
OCA2 gene and eye colour
- OCA2 allele 1: produces a lot of p-protein, allows melanosomes to produce a lot of melanin, brown eyes
- OCA2 allele 2: produces little p-protein, melanosomes produce little melanin, blue eyes
monohybrid crosses
consider one trait and predict the distribution of offspring (distribution of genotypes ≠ distribution of phenotypes)
dihybrid crosses
alleles of 2 genes assort independently (2 different traits are inherited independently of each other if coded on different genes)
incomplete dominance
when an individual is heterozygous with incompletely dominant alleles, the phenotypic expression is a mix of both traits- intermediate expression (pink plants who have 1 red and 1 white allele)
Tay Sachs (prevents the production of lysosomal enzymes- HEX-A)
example of incomplete dominance
- no Tay Sachs: lots of HEX-A protein to break down fats
- mild Tay Sachs: heterozygous- produces enough HEX-A to live normally
- severe Tay Sachs: no HEX-A, fatty structures accumulate in nerve cells and nerves are deteriorated
codominance
codominant alleles are equally expressed in heterozygotes
blood types
example of codominance -type A: IAIA or IAi -type B: IBIB or IBi -type AB: IAIB -type O: ii alleles are the code for enzymes to attach sugar units (antigens) to the surface of RBCs
glycoproteins and ABO blood types
on RBC membranes, ABO antigens differ only by the terminal sugar group attached to the protein (by glycosyltransferase)
- gene on type A, B, AB is functional and codes for enzymes ability to add sugars (type AB codes for 2 types of enzymes)
- type O has a mutated allele and creates a non-functional protein that can’t attach a terminal sugar (no antigens)
XX vs XY chromosomes
XX are homologous, XY are not
X-linked inheritance (recessive)
males need only one affected X to display disorder, females need both affected X to display disorder (females with one affected X and one normal- carrier)
red-green colour blindness
x-linked recessive
polygenic traits
determined by the collective influence of multiple genes/alleles (intermediate phenotype is the most common)
epistasis
a form of non-Mendelian inheritance in which one gene is capable of interfering with the expression of another
(e.g. one gene codes for the production of pigment, another one codes for the deposition of the pigment)
the equation for the number of different genotypes
2^n (n is the number of heterozygous pairs in the genotype)
