The study of genes and the inheritance of traits

CHROMOSOME STRUCTURECHROMOSOME STRUCTURE

Cell

DNA molecule(double helix comprising

genes)

Individual atoms

Histone proteins

Replicated chromosome

Chromatin: a complex of DNA and protein

Genes are part of chromosomes, which are long strands of a chemical substance called deoxyribonucleic acid (DNA).

So genes are made up of DNA. Humans have 46 paired chromosomes (2

pairs of 23), with an estimated 20,000 or so genes that are ‘beaded’ along these tightly bundled strands.

As chromosomes come in pairs, there are two copies of each gene in the cell.

All characteristics are controlled by a minimum of 2 genes.

These genes give the characteristic that may show through.

Different genes that control the same feature are called ALLELES

An Allele is an alternative form of a gene (one member of a pair) that is located at a specific position on a specific chromosome.

Organisms have two alleles for each trait. When the alleles of a pair are heterozygous, one is dominant and the other is recessive.

The dominant allele is expressed and the recessive allele is masked.

Some alleles for a feature are able to mask the influence of other gene when they are paired

The WINNING gene is DOMINANT and is represented by a capital letter e.g. H

The masked gene is RECESSIVE shown as lower case of the same letter e.g. h

T – is considered a dominant allele

t – is considered a recessive allele

TT – is dominant

Tt – is dominant

tt - is recessive

They are the traits determined by reading the genotype.

Example : FF = black fur

Black fur is the phenotype or trait

Uses parental genotypes to determine the possible traits of their offspring.

4 offspring

Polydactylism is a

dominant trait; a

normal number of

digits is the recessive

condition.

Dominant Recessive

Right handedness Left handedness

Hair on middle Segment of digits no hair

Hitch-hiker’s thumb Normal thumb

Polydactylism (extra digits) Normal digits

Brachydactylism (short digits) Normal digits

Pattern baldness Normal hair

Free ear lobes Attached ear lobes

Hitch-hiker’s thumb

Mid-digit hair

Attached ear lobe

Handedness

Free ear lobe

In this crowd of men, almost all

show some degree of pattern

baldness, a dominant trait.

The trait of left or right handedness is genetically determined.

Right-handed people have the dominant allele.

People that consider themselves ambidextrous can assume they have the dominant allele for this trait.

Dominant

Phenotype: Right-handed

Allele: R

Recessive

Phenotype: Left-handed

Allele: r

Determination of eye color is complex, involving perhaps many genes.

Any eye color other than pure blue is determined by a dominant allelethat codes for the production of the pigment called melanin.

Hazel, green, grey and brown eyes are dominant over blue.

Dominant

Phenotype:Brown, green, hazel, or grey

Allele: B

Recessive

Phenotype: Blue

Allele: b

Dominant

The ability to roll the tongue into a U-shape when viewed from the front is controlled by a dominant allele.

There are rare instances where a person can roll it in the opposite direction (to form an n-shape).

Recessive

Phenotype:

Cannot roll tongue

Allele: t

Phenotype: Can roll tongue

Allele: T

Recessive

Phenotype:No hair on mid

segment

Allele: m

Some people have a dominant allele that causes hair to grow on the middle segment of their fingers.

It may not be present on all fingers, and in some cases may be very fine and hard to see.

Dominant

Phenotype:Hair on mid

segment

Allele: M

There is a gene that controls the trait known as hitchhiker's thumb, which is technically termed distal hyperextensibility.

People with the dominant phenotype are able to curve their thumb backwards without assistance, so that it forms an arc shape.

Dominant

Phenotype:Hitchhikers

thumb

Allele: H

Recessive

Phenotype: Normal thumb

Allele: h

Dominant

In people with only the recessive allele (homozygous recessive), ear lobes are attached to the side of the face.

The presence of a dominant allele causes the ear lobe to hang freely.

Recessive

Phenotype: Lobes attached

Allele: f

Phenotype: Lobes free

Allele: F

Dominant Recessive

Curly hair Straight hair

Dark brown hair All other colors

Coarse body hair Fine body hair

Syndactylism (webbed digits) Normal digits

Normal skin pigmentation Albinism

Brown eyes Blue or grey eyes

Short or far-sightedness Normal vision

Normal hearing Deafness

Normal color vision Color blindness

Broad lips Thin lips

Large eyes Small eyes

Roll tongue into U-shape No tongue roll

A or B blood factor O blood factorDark brown hair is dominant over other hair colors

Brown eyes are dominant over blue

Your genetic characteristics (phenotype) *Circle the trait you have!

Number of class mates with each trait (or phenotype). Create a tally.

Eye colourB –brownb – blue

Brown*includes any other eye colour other than blue

Blue Brown Blue

HandnessR – right handr – left hand

Right Left Right Left

Tounge rollT – can rollt – cannot roll

Can roll Cannot roll Can roll Cannot roll

Mid digit hairM – Hairm – No Hair

Hair No Hair Hair No Hair

Thumb extensionH – benth – normal

Hitchhikers Normal Hitchhikers Normal

Ear LobesF – freef – attached

Free Attached Free Attached