Mrs. Degl

Intro to Genetics

Genetics is the branch of biology that deals with patterns of inheritance.

Heredity is the biological process by which parents pass on genetic information to their offspring through their gametes.

Gregor Mendel, an Austrian monk, started the science of genetics by experimenting with pea plants between 1856 and 1868.

Mendel came up with the Principles of Dominance, Segregation, and Independent Assortment.

Mrs. Degl

Based on his findings, Mendel proposed that certain traits were inherited as a result of the transmission of hereditary factors.

Mendel’s hereditary factors, called genes , are located on the chromosomes.

Each gene has a definite position on a chromosome, called a locus.

Mrs. Degl

A gene is a hereditary unit consisting of a sequence of DNA that occupies a specific location on a chromosome and determines a particular characteristic in an organism.

In the early 1900’s T.H. Morgan carried out breeding experiments (crosses) with the fruit fly (Drosophila), that supported Mendel’s findings. He received the Nobel Prize for Physiology or Medicine in 1933.

Mrs. Degl

Two alternate genes that control each trait are called alleles, and they are located on the same positions on homologous chromosomes. (Example: red or brown hair)

Homologous chromosomes are a pair of matching chromosomes in an organism, with one being inherited from each parent.

Mrs. Degl

GENOTYPE = the genes present in the DNA of an organism. 

Pairs of letters are used to represent genotypes for one particular trait. 

There are always two letters in the genotype because (as a result of sexual reproduction) one code for the trait comes from the mother organism & the other comes from the father organism, so every offspring gets two codes (two letters).

PHENOTYPE = how the trait physically shows-up in the organism. 

Mrs. Degl

When we have two capital or two lowercase letters in the GENOTYPE (ex: TT (tall) or tt (short)) it's called

HOMOZYGOUS ("homo" means "the same").  Sometimes the term "PURE" is

used instead of homozygous.

When the GENOTYPE is made up of one capital letter & one lowercase letter (ex: Tt) it's called HETEROZYGOUS

("hetero" means "other").  Just to confuse you, a heterozygous genotype can also be referred to as HYBRID.

Genotype = genes present in an organism (usually abbreviated as two

letters)

TT = homozygous

= pure

Tt = heterozygous

= hybrid

tt = homozygou

s = pure

Mrs. Degl

In genetics, a test cross was first introduced by Mendel in order to determine if an individual exhibiting a dominant trait is a homozygous dominant or a heterozygous dominant. To do this, he crossed a homozygous recessive individual. If all offspring display the dominant phenotype, the individual in question is homozygous dominant; if the offspring are split equally between the dominant and recessive phenotype, the individual is heterozygous.

Test Cross

Mrs. Degl

The Principle of Dominance In a cross of parents that are pure for contrasting traits, only one form of the trait will appear in the next generation.  Offspring that are hybrid for a trait will have only the dominant trait in the phenotype.

Genotype Symbol

Genotype Vocab Phenotype

TT

homozygous DOMINANT

or pure tall

tall

Ttheterozygous

or hybrid

tall

tt

homozygous RECESSIVE 

or pure short

short

Mrs. Degl

T T

t

t

Punnett Squares help us illustrate the crosses that Mendel did.

T = the dominant allele for tall stems

t = recessive allele for short stems

(Let’s fill it in)A summary of this cross would be: 

 

Parent Pea Plants  (P Generation) = Parent

Offspring  (F1 Generation) = First

Generation

Genotypes:

TT x tt

Phenotypes: tall x short

Genotypes: 100% Tt

Phenotypes: 100% tall

Mrs. Degl

The Principle of Segregation During the formation of gametes (eggs or sperm), the two alleles responsible for a trait separate from each other.  Alleles for a trait are then "recombined" at fertilization, producing the genotype for the traits of the offspring.

Mendel now takes two of the "F1" generation (which are hybrid tall: Tt) & crosses them.  He thought he was going to get all tall again (since tall is dominant).  But no! 

T

T

t

t

(Let’s fill it in)

Mrs. Degl

Parent Pea Plants  (Two Members of F1

Generation)

Offspring  (F2 Generation)

Genotypes: Tt x Tt

Phenotypes:

tall x tall

Genotypes: 25% TT 50% Tt 25% tt

Phenotypes:

75% tall 25% short

Mendel’s F1 cross summary:

A helpful thing to recognize:

Any time two parents have the same phenotype for a trait  but some of their offspring look different with respect to that trait,  the parents must be hybrid for that trait. 

Mrs. Degl

The Principle of Independent Assortment Alleles for different traits are distributed to sex cells (& offspring) independently of one another.

So far we've been dealing with one trait at a time.  For example,  height (tall or short), seed shape (round or

wrinkled), pod color (green or yellow). 

Mendel noticed during all his work that the height of the plant and the shape of the seeds and the color of the pods had no

impact on one another.  In other words, being tall didn't automatically mean the plants had to have green pods, nor

did green pods have to be filled only with wrinkled seeds, the different traits seem to be inherited INDEPENDENTLY.

This is called Independent Assortment.

Mrs. Degl

Four o'clock plants have a gene for color and a gene for height with the following phenotypes:

RR: red flower TT: tall plant Rr: pink flower Tt: medium height plant rr: white flower tt: dwarf plant

If a dihybrid plant is self-fertilized, Let’s see proportions of genotypes and phenotypes produced on the next page.

Dihybrid Cross

Mrs. Degl

Number Genotype Phenotype

1 RRTT red-flower, tall plant

2 RrTT pink flower, tall plant

1 rrTT white flower, tall plant

2 RRTt red flower, medium height plant

4 RrTt pink flower, medium height plant

2 rrTt white flower, medium height plant

1 RRtt red flower, dwarf plant

2 Rrtt pink flower, dwarf plant

1 rrtt white flower. dwarf plant

Mrs. Degl

It's like mixing paints, red + white will make pink.  Red doesn't totally block (dominate) the white, instead there is incomplete dominance, and

we end up with something in-between. We can still use the Punnett Square to solve problems involving

incomplete dominance.  The only difference is that instead of using a capital letter for the dominant trait & a lowercase letter for the recessive

trait, the letters we use are both going to be capital (because neither trait dominates the other). 

With incomplete dominance, a cross between organisms with two different phenotypes produces offspring with a third phenotype that is a blending of the parental traits. 

R = allele for red flowers W = allele for white flowers

red x white ---> pink RR x WW ---> 100% RW

Mrs. Degl

R = allele for red flowers W = allele for white flowers

red x white ---> red & white spotted RR x WW ---> 100% RW

With codominance, a cross between organisms with two different phenotypes produces offspring with a third phenotype in which both of the parental traits appear together. 

The genetic gist to codominance is pretty much the same as incomplete dominance.  A hybrid organism shows a third phenotype --- not the usual

"dominant" one & not the "recessive" one ... but a third, different phenotype.  With incomplete dominance we get a blending of the

dominant & recessive traits so that the third phenotype is something in the middle (red x white = pink).

In COdominance, the "recessive" & "dominant" traits appear together in the phenotype of hybrid organisms.