Heredity

Chapter 4

Standard: S7L3

Students will recognize how biological traits are passed on to successive generations.

a. Explain the role of genes and chromosomes in the process of inheriting a specific trait.

Heredity

The passing of traits from parents to offspring.

Traits- The characteristics of an organism.

Alleles

A different form of a gene.

One allele may code for dimples, and the other for no dimples.

Parents

You get one allele from mom, and one from dad.

Genetics

The study of how traits are inherited through the interactions of alleles.

Gregor Mendel

The first to study how traits were passed from one generation to the next.

He studied pea plants.

Mendel’s Experiments

Mendel crossed a pure breeding white flowered plant with a pure breeding purple flowered plant

He got all purple flowersBUT, then he crossed two of the second

generation purples and he got ¾ purple and ¼ white!

What does that mean?

Mendel decided that meant that one alleles was dominant and the other allele was recessive

Dominant vs. Recessive

Dominant Alleles- An allele that covers, or dominates another allele.

Recessive Allele- An allele that is covered, or hidden by another allele.

Punnett Squares

A tool to predict the probability of certain traits in offspring.

Punnett squares show the different ways alleles can combine.

Representing Alleles

Letters! Upper case= dominant alleles. (R)

Lower case =recessive alleles. (r)

Homozygous

If the two alleles that you get from mom & dad are the same, it is homozygous(RR, rr)

Heterozygous

An organism that has two different alleles for a trait is called heterozygous. (Rr)

Genotype vs. Phenotype

Genotype- An organism’s genetic makeup.

Phenotype- Outward appearance, or behavior of an organism.

Example

The tongue rolling allele is R.

R is dominantr is recessive

Your mom is homozygous dominant for the tongue rolling gene.

Her genotype is RR

Your dad is heterozygous for the tongue rolling gene.

His genotype is Rr

Probability

What is the probability that you will be able to roll your tongue?

Do a Punnett Square!

Making a Punnett Square

To set up a Punnett Square, draw a large square, and then divide it into four equal sections.

Dad’s Alleles go on top

Mom’s Alleles go on the side

R

R

R

r

Eggs

Sperm

RR Rr

RrRR

R

R

R

r

Eggs

Sperm

Your genotype could be:•50% RR- Homozygous Dominant•50% Rr- Heterozygous

Your phenotype would be: •100% Tongue Roller•0% No Tongue Roller

Hybrid vs. Purebred

Hybrids=Heterozygous- Have two different alleles for a gene: Rr

Purebreds=Homozygous- Have the same allele for a gene: RR or rr

Let’s Do Another One!

The Big Nose Allele is N.

Big Nose is NLittle nose is n

Mom is Heterozygous for Big Nose: Nn

Dad is Heterozygous for Big Nose: Nn

What is the chance you’ll have a big nose?

Do the cross!

NN Nn

nnNn

N

N

n

n

Eggs

Sperm

What is the genotype?25% NN- Homozygous Dominant

50% Nn- Heterozygous25% nn- Homozygous Recessive

What is the phenotype?

75% Big Nose25% little nose

Incomplete Dominance

When the offspring of two homozygous parents show an “in-between” phenotype.

ExamplePurebred white flowersPurebred red flowersMakes “in-between” color pink flowers

Multiple Alleles

Any trait that is controlled by many alleles.

Blood type is an example.

Polygenic Inheritance

When many genes act to control a trait.

Examples include hair, eye and skin color

Chromosome Disorders

If a sperm or egg cell gets fewer or more chromosomes than normal, this can cause genetic disease.

Down’s Syndrome

These persons have three copies of chromosome #21.

They can be shorter, have learning disabilities and heart problems.

DDD D_

D_DDD

DD

D

D

?

Eggs

SpermDown’s Syndrome

Fetus Dies

Non-disjunctionWhen chromosomes fail to separate.

One sex cell gets an extra chromosome.

One sex cell doesn’t get a chromosome.

Recessive Genetic Disorders

Offspring only gets the disease if he/she inherits the recessive alleles from mom and dad.

Cystic Fibrosis

Causes a thick fluid to build up in the lungs, making it difficult to breathe.

Cystic Fibrosis is a Recessive Genetic Disorder.

Carriers

Mom and dad are carriers- They carry the defective allele, but don’t have the disease.

CC Cc

ccCc

C

C

c

c

Eggs

Sperm

Cystic Fibrosis

Sex Determination

Females have XX chromosomes.

Males have XY chromosomes.

XX XY

XYXX

X

X

X

Y

Eggs

SpermGirls

Boys

Sex-Linked Disorders

An allele inherited on a sex chromosome.

Color blindness is an example.

Selective Breeding

Selecting plants or animals with the most desirable traits to breed for the next generation.

Genetic Engineering

Finding desirable genes in one plant, and inserting them into another plant.

This can make vegetables bigger, heartier and more resistant to insects and fungi.

Genetic Anomalies