Single Gene InheritanceChapter 2

- the two members of an homologous pair carry alleles for the same genes and, therefore, affect the same traits.

Gene are located on chromosomes

Chromatin – 25% protein + 75% DNAChromatin condenses

into chromosomes at cell division

The stuff that makes up chromosomes

Packaging of DNA

Helix Nucleosome Fibers Loops

Loops Coils Chromosome

A chromosome, A closer view

DefinitionsPhenotype - the visible expression of

information contained in the genetic make-up of an individual

Genotype - the genetic make-up, latent or expressed. The sum of all genes present in an individual.

P1 - the parental lines

F1 - the first filial generation, the offspring resulting from a cross. F2/ F3 the second and third generations.

Genetic polymorphismsThe stuff of genetic variation

1 locus (monogenic), 2 different alleles 3 combos possible

Mendel’s Principles of Inheritance

1) Genetic characteristics are controlled by unit factors (elemente) that exist in pairs in individual organisms.

These unit factors represent units of inheritance today called genes.

Alternative forms of a single gene are called alleles.

Individuals can have identical alleles (homozygous) or different alleles (heterozygous) for a single gene.

An example: seed color

+o oX P1 (parental)

F1

Self-pollination of the F1

+o oX

3 1

Dominance/Recessiveness

When two unlike alleles responsible for a single character are present in a single individual, one is dominant (expressed) to the other which is said to be recessive (silent).

+o oXGG gg

P1

Gg

F1

Segregation

During formation of the gametes, the paired alleles separate or segregate randomly so that each gamete receives either with equal likelihood.

G

G

g

g

Possible egg alleles

Gg

GgGG

gg

Punnett square

Possible sperm alleles

Sample Problem

• Flower position in pea plants is another single gene trait. Axial flower position is dominant to terminal. If we cross a true breeding plant with axial flowers to one with terminal flowers, what phenotypes can we expect in the F1 progeny? in the F2 of a selfing of the F1?

Sample Problem

• Having dimples is a dominant trait in humans. I have no dimples but my father and mother do. All my siblings have dimples as well. Am I adopted??

More on Single Gene Inheritance

P1 red X white

F1 pink

Query

Self F1 pink X pink

red pink white 1 2 1

F2

Self F1 pink X pink (R1RR22) ) (R1RR22))

Incomplete (partial) dominance

R1 RR22

Possible sperm cells

Possible egg cells

R1

RR22

R1RR22

pinkpink

R1RR11 R1RR22

pinkpink

R2RR22

Intermediate expression of the phenotype.

But what about the yellow ?

This could be the effect of multiple alleles involved in color production.

Multiple alleles

An example: the ABO blood types in humans

Genotype Phenotype

IAIA

IAIO

IBIB

IBIO

IAIB

IOIO

A

B

AB

O

Sample problem

• Red-green color blindness is a sex linked, recessive allele of a single gene trait. My brother-in-law is red-green color blind. My sister is not, but my father might have been. My sister and her husband have 4 sons. My mother (their grandmother) was worried that they would be color blind. Should she have been concerned?

Genes can be Pleiotropic

-single gene, multiple effects

ex. dwarf

Chromosomal Basis of Inheritance

Meiosis (the prelude to

sexual reproduction)

For sexual reproduction to occur, chromosomes must be duplicated and divided between the gametes.

Meiosis I

Meiosis II

Meiosis

Each gamete contains only one member of each homologous pair.

There are 2 steps of meiosis - 2 cell divisions, but only

1 replication of chromosomes.

Summary

Mendel’s “rules of the game”

3) Segregation- during formation of the gametes, the paired unit factors separate or segregate randomly.

1) Genes occur in pairs - Genetic characteristics are controlled by genes that exist in pairs.

2) Dominance/Recessiveness- When two unlike alleles responsible for a single character are present in a single individual, one is dominant (expressed) to the other which is said to be recessive (silent).

Discovering genes via Mutant analysis

• Generating mutants

– Chemical mutagenesis (EMS)

– base transition, point mutation

– Radiation

– deletions

– Transposons/ T-DNA tags

– insertion/deletions (indels)

And observing segregation ratios

• Planned crosses and Punnett squares, Pedigree analysis

– Mutant or polymorphism analysis• Autosomal dominant/recessive

• Sex-linked genes