Genetic Processes: Mendelian Genetics

SBI3U (Secours) ⚠️ for some reason, the superscripts sometimes appear as subscripts, pretend they're superscripts

Gregor Mendel

• the father of genetics

• Austrian Botanist Monk

Mendel conducted experiments on what and when?

conducted experiments regarding the traits of pea plants, before knowledge of DNA and chromosomes

Monohybrid Cross

a cross designed to study the inheritence of only a single trait

true-breeding

purebred, refers to offspring that always carry the same trait as the parent

Factors

old term used by Mendel, replaced by “allele”

Allele

a form of a gene, each cell has two per gene

Homozygous individual

carries two of the same alleles

Heterozygous individual

carries two different alleles

Genotype

genetic make-up of an individual

Phenotype

an individual’s physical appearance with respect to a specific characteristic

Punnett Square

diagram that shows all possible allele combinations from the parents, used to determine the probability of an offspring’s genotype

Mendel originally thought that crossing true-breeding purple pea flowers with true-breeding white flowers would produce ____ coloured flowers (a blend). He was _____, instead, ___% produced ______ coloured flowers.

lavender, wrong, 100, purple

#1 Principle of Dominance

• there are dominant and recessive traits, in which alleles dominate over others

• if two homozygous individuals are crossed, all the offspring will be identical for phenotype and genotype

In Mendel’s 2nd experiment, he allowed plants of the F1 generation to ________. The resulting F2 generation always had the ________ ratio of ______% purple _______% white. The _____ ratio was 25% ____, 25% ____, and 50%____.

self/auto-pollinate, phenotype, 75%, 25%, genotype PP, pp, Pp

#2 Principle of Segregation

• organisms inherit two copies of genes, one from each parent

• organisms donate only one copy of each gene to their gametes due the genes being separated in meiosis

Sex determination in humans (% chances)

50% chance male and 50% chance female

Sex determination in other animals

• birds - mother’s chromosomes determine sex

• turtles - temperature determines sex

• ants - fertilized=female

• clownfish - age determines/changes sex

• certain lizards - sex is always female

How to determine the probability that a couple will have 𝑥 amount of boys/girls in a row?

½ to the power of 𝑥

Test Cross

a cross used to determine the genotype of an individual expressing a dominant trait

Why are test crosses necessary? How do they work?

• by appearance alone, it’s impossible to know if the individual is showing a homozygous or heterozygous dominant trait

• to do a test cross, the unknown genotype is crossed with a homozygous recessive genotype

• the offspring are then examined to determine the unknown genotype (what matches the phenotype of the offspring best)

In Mendel’s 3rd experiment, he wanted to know if inheritance of a characteristic influenced the inheritance of another characteristic. The characteristics he experimented with were seed colour (____&____) , and seed shape (_____&______)

Yellow (Y), Green (y), Round (R), Wrinkled (r)

#3 Law of Independent Assortment

• if genes are located on separate chromosomes, they will be inherited independently of one another

• during meisosis I, chromosomes align and separate independently of those from other tetrads (no mixing of characteristics)

• this allows for formation of gametes which would otherwise be impossible/not present

Dihybrid Cross

a type of cross involving two pairs of heterozygous alleles

how do you use a punnett square for dihybrid crosses

always use FOIL first to determine the gametes

a dihybrid cross of the F1 generation produces how many different genotypes in the F2 generation?

9

what is the phenotypic ratio of F2 generation?

9:3:3:1

Incomplete Dominance

a situation where allele will determine the phenotype regardless of the presence of another allele

⇨ blend of two homozygous phenotypes

examples of organisms and characteristics that use the pattern of incomplete dominance

roses, curly hair, skin colour, fur colour of certain animals (ex: horses)

in the red and white flower example, how are incomplete dominance allele written?

colour indicated by superscript: C^R and C^W

if a red and white snapdragon flower (incomplete dominance) are crossed, what is the resulting F1 generation’s genotype and phenotype?

100% C^RC^W , meaning 100% pink flowers

Lethal Alleles

alleles that cause death at the embryonic stage

if two creeper (Cc) chickens mate, what are their phenotypes?

25% lethal, 25% normal, 50% creeper

if a creeper and a normal chicken mate, what are their phenotypes? Do lethal alleles cause death in any embryo? why?

• 50% normal, 50% creeper

• because only one lethal allele is present, it cannot be homozygous therefore it doesn’t cause death

Multiple Alleles

existence of 3 or more alleles for the same characteristic (ex: blood type)

What are the 3 alleles possible for blood types?

I^A (dominant), I^B (dominant), ii (recessive)

What do the blood type alleles refer to?

the types of antigens (proteins) found in red blood cells.

⇨ type A has antigen A, type B has antigen B, type AB has both A and B antigens, and type O has no antigens

What is the universal donor blood type?

type O

What is the universal blood recipient?

type AB

What blood type is the most common in Canada? What type is the least common?

O is most common, AB is the least common

Co-dominance

phenomenon where the phenotypes of two different alleles are displayed simultaneously.

Incomplete Dominance vs Co-dominance

incomplete dominance results in a mix of traits, while codominance results in both traits being expressed separately

Polygenic Inheritence

Characteristics that are expressed by multiple genes

ex: eye colour, skin colour