Biology 30 - Genetics

Aristotle

384-322 BCE

proposed pangenesis to explain why offspring resemble their parents

states that egg and sperm contain tiny particles (pangenes) from all parts of the body

upon fertilization, the pangenes develop into the parts of the body they derived from

Antony van Leeuwenhoek

1632-1723

proposed homunculus - complete, miniature person in the head of a sperm which came from the father but developed in the mother

Regnier de Graaf

proposed that the egg contained the entire person

sperm only stimulated the egg to develop

Genes

sequences of DNA that contain the instructions to make different proteins

two copies of every gene from parents: 1 maternal copy, 1 paternal copy

Alleles

different versions of genes

dominant: uppercase

recessive: lowercase

Genotype

genetic makeup of a person

Phenotype

what we actually/physically see in the person

Homozygous

matching alleles

purebred

Heterozygous

non-matching alleles

hybrid

Punnett squares

predicts genotypes and phenotypes of offspring of different crosses

Grigor Mendel

Augustinian monk

1822-1884

bred and analyzed over 28 000 pea plants in 7 years

laws formed the foundation of modern science of genetics

studied the inheritance of 7 traits that were each expressed in 2 easily distinguishable forms

Genetic cross

combination of 2 gametes from the P (parental) generation

First filial generation

F₁

genetic material of the P gametes combine in different ways

Monohybrid cross

genetic cross that involves only 1 trait

Mendel pea shape examples

first let plants self-pollinate to ensure the plants were purebred; exhibited characteristics generation after generation

crossed pure breeding plants (P generation) with opposite traits (e.g. purple flowered plants w/ white flowered plants)

results in F₁ all being purple

F₁ plants pollinate each other → creating F₂ generation

What is special about the F₂ generation of a monohybrid cross of dominant and recessive purebreds?

ratio is always 3:1 (dominant: recessive phenotype)

Law of Segregation

Mendel’s first law

all individuals have 2 copies of each gene → these segregate randomly during gamete formation and each gamete receives only 1 copy of each gene

when two gametes fuse during fertilization, an entirely new combination of parental genetic material is created (variation within a population)

Incomplete dominance

when 2 alleles are equally dominant

interact to produce a new phenotype

NOT simple dominant and recessive

Sickle-Cell Anemia

incomplete dominance

normal haemoglobin allele = Hb^A

sickle-cell haemoglobin allele = Hb^S

Hb^AHb^A: normal

Hb^AHb^S: sickle-cell trail (carrier)

Hb^SHb^S: sickle-cell disease

heterozygous → malaria resistance = advantageous

Codominance

both alleles fully expressed → individual expresses BOTH phenotypes at once

Multiple alleles

in many organisms, most genes have more than 2 alleles (in the population as a whole)

human blood types follow this pattern: 3 alleles → 4 blood types (phenotype)

Rule of independent events

chance of an event occurring is UNAFFECTED by previous results

Rule of sums

“or” = add the probabilities

chance of one or the other event occurring = SUM of these two events happening separately

Rule of products

“and”: multiply the probabilities

chance of two events occurring at the same time = PRODUCT of these two events happening separately

Dihybrid crosses

2 traits

4 symbols/letters in the genotype of 1 organism

Law of independent assortment

Mendel’s second law

segregation of alleles of one gene has NO INFLUENCE on segregation of alleles of another

the two alleles for one gene segregate (assort) independently of the alleles for the other genes during gamete formation

What is the phenotypic ratio of a dihybrid cross of heterozygous parents?

according to Mendel → will ALWAYS 9 : 3 : 3 : 1

9: dominant for trait 1 & 2

3: dominant for trait 1 & recessive for trait 2

3: recessive for trait 1 & dominant for trait 2

1: recessive for trait 1 & 2

Are dominant alleles always better?

not always

with some inherited traits, recessive alleles benefit the organism more than if they were to receive the dominant allele

Huntington’s Disease

inherited disease

neurodegenerative disorder

allele for it is dominant

homo or heterozygous = develop HD

Achondroplasis Dwarfism

typically inherited

homozygous dominant

heterozygous individuals = have this form

no homozygous individuals as it would be lethal

Lethal alleles

detrimental effect → organism dies

exhibit incomplete dominance → only homozygous individuals die from two alleles

affected individuals do not reach reproductive age

Tay-Sach’s Disease

lethal homozygous recessive

Why do lethal allele traits still exist if those affected with 2 alleles die before reproducing?

allele passed through heterozygotes

Epistasis

masking effect

allele at one locus (gene location) prevents the allele at another locus from being expressed

extension of dominance concept for alleles within the same homozygous pair (i.e. locus)

Chromosomal theory of inheritance

1902 - Walter Sutton and Theodor Boveri independently conclude that chromosomes carry genes

movement of each pair of homologous chromosomes is independent of movement of all other pairs of homologous chromosomes → follows Mendelian inheritance patterns

genes that are carried on the same chromosome do not assort independently → does not follow Mendelian inheritance patterns

Fruit fly genetics

Thomas Hunt Morgan set out to test Sutton-Bovery theory using fruit flies as they reproduce at a very fast rate

crossed a white-eyed male with a red-eyed female

F1 generation all had red eyes → red dominant to white

crossed male and female from F1 generation → expected phenotypic ratio is 3 (red eyes) : 1 (white eyes)

actual results: 100% of females had red eyes, 50% of males had white eyes

therefore eye colour connected to gender → gene must be located on the X chromosome

Results & analysis of fruit fly genetics

female fruit flies have 2 X chromosomes

male fruit flies have 1 X chromosome & 1 Y chromosome

F1 data indicates white-eyed trait is indeed recessive → males only have one X chromosome so only need one recessive allele to have white eyes

Barr body

in every human female cell, one of the X chromosomes is inactivated

the barr body is the condensed structure of the inactive X chromosome

Mendel’s work

genetic traits (i.e. genes) assort independently from one another

Sutton-Boveri’s work

when alleles of two different genes are on the same chromosome, they do not necessarily segregate independently

called linked genes

Crossing over

point at which a crossover occurs b/w two genes, alleles will be on separate chromosomes → alleles will migrate to different gametes

random event

occurs at any point on sister chromatids except near centromere

more likely to occur b/w genes that are farther apart on a chromosome than b/w genes that are closer together

Morgan’s work

any given pair of linked gene would separate with a predictable frequency

results could be explained by assigning each gene a specific position along a linear chromosome

Morgan and his students were able to amend the chromosomal theory of inheritance → gene-chromosome theory

Gene-chromosome theory

genes exist at specific sites arranged in a linear manner along chromosomes

What is crossing over used to determine?

relative positions of genes on a chromosome

One map unit

distance between points on a chromosome where a crossover is likely to occur in 1% of all meiotic events

1% = 1 map unit = 1 mu

Recombinant frequency

F1 offspring that have the same pheno/genotype as parents = parentals

F1 offspring that have different pheno/genotypes than parents = recombinants

% of recombinant types in F1 generation is directly proportional to the distance between two gametes

greater distance = greater likelihood of crossover event occurring

Recombination frequency

percentage of times that crossover occurred as P gametes were formed → if <50% → genes are linked

formula: # of recombinant types/total # of offspring x 100%

Model subjects

fruit flies were chosen as test species due to their fast rate of reproduction, large number of offspring, were considered more ethical, and ability to select desired traits

Human pedigrees

collect as much information about a family’s history available, and use this information to create a flowchart that uses symbols to show patterns of relationships and traits in a family over many generations

unethical to perform experimental crosses b/w selected men and women

unethical to accumulate large numbers of offspring from the same human parents

Autosomal dominant inheritance

number of men = number of women (affects both equally)

does NOT SKIP generations (generally appears in ALL GENERATIONS)

traits that are carried on autosomes

both homozygous dominant and heterozygous will exhibit trait

Autosomal recessive inheritance

number of men = number of women (affects both equally)

SKIPS generations

only homozygous recessive will exhibit trait

tend to appear more frequently in families with consanguineous marriages (inbreeding)

X-linked dominant traits

increased number of women affected

does NOT SKIP generations

typically seen in females (often lethal in males)

• only need one dominant allele to be affected

less common than X-linked recessive disorders

X-linked recessive traits

increased number of men affected

SKIPS generations

affects men more than women

females must have both recessive alleles to be affected, males only need one

heterozygous females are unaffected → carriers

no father to son transmission

Y-linked traits

ONLY in men (on Y chromosome)

does NOT SKIP generations in males

if father affected → sons will ALWAYS be affected