Genetics

What accounts for the passing of genetic traits from parents to offspring?

Are traits blended in the offspring? Or: are traits inherited as single, discrete

and separate units (genes)?

Today, much of what we know about genetics and heredity started with the work of an Austrian monk in the 1800s—Gregor Mendel

Mendel discovered the basic principles of heredity with his experiments with garden peas.

Fig. 14-1

Varieties include those with distinct heritable characteristics (traits) such as flower color or plant height.

Pea plants are normally self-pollinated, but can be easily cross-pollinated by the plant breeder.

Therefore, the breeder can control which traits are crossed.

Pea Flowers have Petals that are closed over the Stamens and Carpels. They are not open to wind and other pollinators such as insects. Therefore, they are self-pollinating. What does this mean regarding their genetic diversity?

Mendel chose traits that were either..or He also chose plants that were true-

breeding for a particular trait. (Purebred). And, since peas plants are normally self-

pollinated, he knew what genes they carried.

In his experiments, Mendel crossed two plants with contrasting traits, producing offspring called hybrids.

The true-breeding parents are the P (parental) generation

The offspring are called the F1 generation.

When the F1 generation plants self-pollinate, their offspring are called the F2 generation.

P

• When Mendel crossed contrasting, true-breeding white and purple flowered pea plants, all of the F1 hybrids were purple

• When Mendel crossed the F1 hybrids, many of the F2 plants had purple flowers, but some had white

• Mendel discovered a ratio of about three to one, purple to white flowers, in the F2 generation

Fig. 14-3-3

EXPERIMENT

P Generation

(true-breeding parents) Purple

flowers Whiteflowers

F1 Generation

(hybrids) All plants hadpurple flowers

F2 Generation

705 purple-floweredplants

224 white-floweredplants

In the first experiment, when only purple flowers were produced, Mendel thought that the white trait had possibly been absorbed and had disappeared.

However, when it reappeared in the F2, he knew it had just been hidden.

So, he called the purple colorthe dominant trait and thewhite color the recessive trait.

Table 14-1

• Mendel developed a hypothesis to explain the 3:1 inheritance pattern he observed in F2 offspring

• Four related concepts thatmake up this model can be related to what we now know about genes and chromosomes

Mendel’s garden in the abbey in Austria where he conducted his experiments.

Alternative versions of genes account for variations in traits.

For example: there are two versions of the gene for flower color in peas: purple and white.

These alternative versions are called alleles—each of which resides in a particular place on a chromosome (the locus).

For each characteristic, an organism inherits two alleles: one from the male parent and one from the female parent.

The alleles may be the same (like the true-breeding plants), or they may be different (like the F1 hybrids).

Mendel figured all of this out without ever knowing anything about chromosomes!

HomozygousAxial

Homozygous terminal

Heterozygous Axial

If the two alleles at a locus are different, then one of them (the dominant allele) determines what the organism will look like.

The recessive allele has no noticeable affect on the appearance.

Why are all of the F1 offspring purple?

The law of segregation states that the two alleles for a heritable character separate (segregate) during gamete formation and end up in different gametes

An egg or sperm gets only one of the two alleles that are present in the somatic cell. Mendel figured this

out without ever knowing anything about meiosis!

How do we explain the 3:1 results that Mendel got in the F2 generation?

If we know the genetics of the parents, a Punnett Square can show the possible combinations of genes the offspring can inherit.

Use CAPITAL letters for dominant genes, lower case letter for recessive

Homozygous—an organism with two alleles for a given trait that are identical (also called purebred)

Heterozygous—an organism with two alleles for a given trait that are different. (these are also known as hybrids)

An organisms’ traits may not always reveal its true genetics. (think about those hybrid purple plants)

Phenotype: what the organisms’ physical traits are—what it looks like

Genotype: what genes the organism carries—what are its actual genes?

Notice that the Genotype Ratio is 1:2:1 and the Phenotype Ratio is 3:1.Why are they different?

TECHNIQUE

RESULTS

Dominant phenotype, unknown genotype:

PP or Pp?

Predictions

Recessive phenotype, known genotype: pp

If PP If Ppor

Sperm Spermp p p p

P

P

P

p

Eggs Eggs

Pp

Pp Pp

Pp

Pp Pp

pp pp

or

All offspring purple 1/2 offspring purple and1/2 offspring white

Test CrossTechnique

Mendel figured out the Law of Segregation by studying the results of crosses involving only one trait (for example, flower color).

These are called monohybrid crosses.

Mendel derived his second law by studying crosses involving pea plants who differed in two traits. These are called dihybrid crosses.

An organism who is hybrid for two traits is called a dihybrid.

A dihybrid cross, a cross between F1 dihybrids, can determine whether two characters are transmitted to offspring as a package or independently

1. Figure out the genotypes of the parents.

2. Figure out the possible combinations of genes that could be in the gametes of these parents. Example: TtYy could produce 4 kinds of gametes: TY, Ty, tY, ty

3. Put the gametes into a Punnett Square & Solve

4. Figure out genotypic and phenotypic ratios.

• Using a dihybrid cross, Mendel developed the law of independent assortment

• The Law of Independent Assortment states that each pair of alleles segregates independently of each other pair of alleles during gamete formation

Law of Independent Law of Independent AssortmentAssortment

Remember that in Meiosis, it is random which direction the chromosomes go during Anaphase I and II. There are all kinds of possibilities!

Inheritance of characters by a single gene may deviate from simple Mendelian patterns in the following situations: When alleles are not completely dominant or

recessive When a gene has more than two alleles When a gene produces multiple phenotypes

Why does everything have to be so complicated????

Complete dominance—when the dominant gene totally dominates over the recessive.

Most of the traits Mendel studied showed complete dominance.

In incomplete dominance, the phenotype of F1 hybrids is somewhere between the phenotypes of the two parental varieties

Red

P Generation

Gametes

WhiteCRCR CWCW

CR CW

F1 GenerationPinkCRCW

CR CWGametes1/21/2

F2 Generation

Sperm

Eggs

CR

CR

CW

CW

CRCR CRCW

CRCW CWCW

1/21/2

1/2

1/2

Incomplete Dominance in Snapdragons

In codominance, two dominant alleles affect the phenotype in separate, distinguishable ways

There isn't a blending of the traits, but rather both alleles are present in the phenotype.

In this flower, both the dark pink allele and the white allele are co-dominant. Neither one dominates over the over, so the phenotype shows both alleles.

Polygenic Traits are characteristics that are affected by more than one gene.

Examples: Eye Color, Skin Color Eye color comes from different genes which affect

tone, amount and position of the pigments. Skin color is determined by at least 3 different genesworking together to produce a wide variety of tones.

Most genes exist in more that two allelic forms.

A classic example of this is human blood types.

Four major blood types exist: O, A, B, and AB

They are named for the presence or absence of certain carbohydrates on the surface of the red blood cells.

The gene for blood type has 3 possible alleles: IA, IB, and i

Both IA and IB are dominant over i

IA and IB are co-dominant to each other

Type A Blood Genotypes: IAIA or IAi

Type B Blood Genotypes: IBIB

or IBi Type AB Blood

Genotype: IAIB

Type O Blood Genotype: ii

Pedigrees are family trees used to describe the genetic relationships within a family.

Pedigrees can be used to determine the risk of parents passing certain conditions to their offspring