Biology - Fundamentals of Genetics

Genetics

Field of biology devoted to understanding how characteristics are passed from parents to offspring

Heredity

The transmission of characteristics from parents to offspring

Gregor Mendel

Studied the heredity of garden peas through seven characteristics of peas plants (father of genetics); Monk that took care of the garden and had degrees in math (statistics) and science

Garden Peas

Had a lot of offspring generated quickly, could control their mating, and were easy to see their traits

Trait

Genetically determined variant of a characteristic

* purple vs. white flowers

What did Mendel notice when he crossed traits?

One trait was dominant over the other and one showed more in the offspring

* More purple than whit flowered plants

What did Mendel notice would sometimes happen was crossing parent plants that both had dominant traits?

He noticed that the weaker (recessive) traits would show up sometimes, even if both of the parents had the dominant trait

* Purple x Purple plants produced mostly offspring plants with purple flowers but sometimes would produce white flowered offspring

Pollination

occurs when pollen grains produced in the male reproductive parts of a flower, called the anthers are transferred to the female part if the flower called the stigma

Self-pollination

Pollen is transferred from the anthers of a flower to the stigma of either that flower or another flower on the same plant

Cross-pollination

Pollination between flowers of two plants

How was self-pollination prevented?

by removing the anthers of a plant

How was cross-pollination preformed?

by manually transferring the pollen from the flower of a second plant to the stigma of the anther-less plant

True (pure) - breeding

Plants that always produce offspring with the same traits when they self - pollinate

* Purple flowers plants that produce purple flowered offspring

P generation

True breeding parents that Mendel cross - pollinated

* Planted the seeds and all offspring had purple flowers

F 1 generation \[1st filial (son in Latin)\]

Offspring of the P generation

F 2 Generation

Offspring produced when F 1 generation was allowed to self - pollinate

* Most purple offspring but some white (3:1)

Factors

were controlling the plant characteristics (according to Mendel)

* Each trait (purple and white flowers) were controlled by a separate factor \[two (paired) overall factors controlling traits\]

Genes

appear on homologous chromosomes and are the correct term for the “factors” Mendel observed

Law of Segregation

States that a pair of factors (genes) is segregated or separated, during the formation of gametes

* Each parent only gives one gene out of the two genes they have to their offspring

Meiosis

%%separates%% the genes %%independently%% since they are on different chromosomes

* Genes on the same chromosomes don’t separate independently since they are connected (will get different results)

Law of Independant Assortment

States that factors (genes) controlling different characteristics separates independently from one another during the formation of gametes

* Each parent gives genes for different traits separately from each other

Allele

form of a gene designed by a letter

* “P” for purple flower allele (dominant)
* “p” for white flower allele (recessive)

Genotype

The two alleles that an organism inherited

* PP
* Pp
* pp

Phenotype

The appearance of the organism

* “PP” or “Pp” mean purple
* “pp” means white

Homozygous

When the two alleles in a pair are alike

* “PP”
* “pp”

Heterozygous

When the two alleles in a pair are different

* “Pp”

Probability

The likelihood that a specific event will occur

* coin toss: 1/2 → 0.5 → 50% → 1:1
* dice roll: 1/6 → 0.17 → 17% → 1:5

Equation for probability

Number of times an event is expected to happen / number of times an event could happen

Rule of Multiplication

Chances that two separate events will happen at the same time

* 2 coin toss: (both coins land heads) 1/2 X 1/2 = 1/4

Rule of Addition

Chances that an event can happen in two or more alternative ways

* 2 coin toss
* (heads, tails) 1/2 X 1/2 = 1/4
* (tails, heads) 1/2 X 1/2 = 1/4
* 1/4 + 1/4 = 2/4 = 1/2

Monohybrid cross

Cross in which only one characteristic is predicted

Punnet Square

Diagram that predicts the possible traits of the offspring

Test cross

An individual of unknown genotype is crossed with a homozygous recessive individual to try and find out the unknown genotype

Have a black mouse that does not know if it has “BB” or “Bb” as it’s genotype. Cross it with a brown mouse (“bb”)

Incomplete Dominance

Offspring will have a phenotype in between that of the parents

* Snapdragon plants
* Alleles:
* R - red
* W - white
* Genotypes - Phenotypes
* RR - red flowers
* RW - pink flowers
* WW - white flowers
* Hair
* Alleles
* S - Straight
* C - curly
* Genotypes - Phenotypes
* SS - straight
* SC - wavy
* CC - curly

Codominance

Both alleles for a gene are expressed in a heterozygous offspring

* Hair
* Alleles
* R - Reddish brown hair
* W - white hair
* Genotypes - Phenotypes
* RR - reddish brown hair (bay coat)
* WW - white coat
* RW - reddish brown and white hairs (roan coat)

Codominance in blood

* Alleles
* A - A protein allele
* B - B protein allele
* o - no protein allele (recessive)


* Genotypes - Phenotypes
* AA, Ao - A proteins (Type A)
* BB, Bo - B proteins (Type B)
* AB - A and B proteins (Type AB)
* oo - no proteins (Type o)

Sex linked (x linked)

Genes that are located on the x chromosome

Recessive diseases inherited on the x chromosome

Red-Green Colorblindness

* defective cone cells at back of eye

Hemophilia

* Excessive bleeding due to defective clotting factor

Muscular dystrophy

* deterioration of muscle over time

The effect of sex linked diseases on men

It is easier for males to inherit these conditions since they only need one copy of the recessive disease allele

male genotype

XY

female genotype

XX

Polygenic inheritance

A single characteristic may be influenced by many genes

* Skin color is a combination of 3 sets of genes that interact to determine skin color.
* This is also an example of incomplete dominance.
* Would need to perform a trihybrid cross to figure out the possibilities

Family Pedigree

Tool to track genetic traits/disorders

In what kind of groups are genetic disorders most likely separated?

Most human genetic disorders are more common in certain ethic groups since genes would be randomly created and stay within a certain population of humans that were isolated from others

What are most genetic disorders?

Recessive (like albinism, cystic fibrosis, phnylketonuria (PKU), Tay-Sachs disease.

* This makes carriers possible

inbreeding causes what? how?

Diseases show up more in offspring if there is inbreeding (mating of close relatives) since there is a higher likelihood that both parents are carriers.

What are some genetic disorders?

Some genetic diseases are caused by dominant alleles/genes

* They are less common since dominant diseases are usually serious and people die before they reproduce
* no carriers
* Achondroplasia (dwarfism), Huntington’s disease

Pleiotrophy

A single gene may affect many phenotypic charateristics

Sickle cell anemia

* An example of pleiotropy and codominance


* 1/10 African-Americans are carriers
* improves resistance to Malaria in carriers (parasites carried by blood sucking bugs that harm red blood cells)
* Malaria causes cells to sickle
* Sickle cells in carriers are destroyed by the body along with the parasite (immune system reds the body of these) and the parasite does not grow well in the normal cells that remain
* Carriers exhibit some symptoms (codominace situation) due to having both normal and sickle cells at the same time

Dihybrid crosses

Cross in which two characteristics are predicted

* A black mouse, with short hair is crossed with another black mouse with short hair. Both parents are heterozygous for both traits. what will the potential babies look like in terms of color and hair length?