Genetics Since

Mendel

GLE 0707.4.4

Predict the probable appearance of

offspring based on the genetic characteristics

of the parents.

What You’ll LearnExplain how traits are inherited by incomplete dominanceCompare multiple alleles and polygenic inheritance, and give examples of each.Describe two human genetics disorders and how they are inherited.Explain how sex-linked traits are passed to offspring.

Why it’s important!Most of your inherited traits involve more complex patterns of inheritance than Mendel discovered.

I. Incomplete Dominance

When a pure-bred organism is crossed with another pure bred organism, an intermediate phenotype is created.

The offspring of two homozygous parents – incomplete dominance

examples of this dominance include flower color of some plant breeds and the coat color of some horse breeds.

Figure 5

Chestnut horse

Cremello horse

Palomino horse

A. Multiple AllelesMany traits are controlled by more than two alleles.

This trait is said to be controlled by multiple alleles.And, produce more than three phenotypes of that trait.

Example of Multiple alleles.

Blood type example

https://www.youtube.com/watch?v=ZU0uiuqUYng

3.33 minutes

II. Polygenic Inheritance

Polygenic inheritance – when a group of gene pairs acts together to produce a trait

It produces a wide variety of phenotypes Example – eye color (lots of genes),skin (3 to 6 genes), height and hair.

The environment – plays a role in expression of traits controlled by polygenic inheritance.

It is common in grain color in wheat, milk production in cows, and egg production in chickens.

https://www.youtube.com/watch?v=gouqTq5p168 - 3 minutes

A. Impact of the environment• Environmental influence

can be internal or external.• Ex. Male birds are

colorful, not the females.

• A chemical in their body determines the expression of color.

• Some people have genes that make them at risk for developing certain cancers • Skin cancer genes

– limit exposure to sun and take care of skin. A person may never get skin cancer.

Figure 6 –

Himalayan rabbits have alleles for dark-colored hair.This allele is able to be express in low temperatures- only the ears, nose, feet and

tail have dark-colored hair.

III. Human Genes and Mutations

Sometimes a gene undergoes a change-trait is expressed differently.

Errors occurs in DNA when copied causing mutations.

Certain chemicals can produce mutation in plants or animals, including humans; X-rays and radioactive also causes it.

A. Chromosomes DisordersProblems can occur in incorrect number of chromosomesIn humans, it is usually fatal to unborn embryo or baby after birth.• Ex. Down syndrome (three copies of chromosome 21

are in fertilized egg)• Individual with DS – short, exhibit learning

disabilities and heart problems.

IV. Recessive Genetic Disorders

Some recessive genes result in mutations within the gene.

Both parents carry the recessive gene (heterozygous); therefore no symptom show.

They can pass on the recessive allele to the child and he or she will have a recessive genetic disorder.

Reading Check: How is cystic fibrosis inherited?

Cystic Fibrosis (production of extremely thick mucus) – homozygous recessive disorder commonly found in Caucasian Americans.

Results in repeated bacterial respiratory infectionsPhysical therapy, special diets, and new drug therapies increase life span of patients.

V. Gender Determination

Genetics used labels when studying organism-

X – female egg ; X or Y – male sperm

Fertilization of both sex cells - causes offspring

Female XX and males offspring XY

Sometimes chromosomes don’t separate during meiosis, and the individual can inherit an abnormal number of sex chromosomes.

https://www.youtube.com/watch?v=Nrv5yqEMQ4Q 3 min.

Figure 8How do the X (left) and Y (right) chromosomes differ from one another in shape and size?

VI. Sex-linked Disorders

A sex-linked gene is an allele on a sex chromosome. Some conditions from inheriting this gene is called a sex-linked disorder.Ex. Red-green color blindness-are found on X chromosomes

People have trouble seeing difference between red and green and sometimes yellow. (It is a recessive sex-linked disorder)

Dominant sex-linked disorders are rare and result when a person inherits at least one dominant sex-linked allele.

Vitamin D resistant Rickets – X linked dominant disorder

https://www.youtube.com/watch?v=8OGA4rGDk3M 6:52 min

VII. Pedigrees Trace Traits

A pedigree is a visual too for following a trait through generations of a family.

Males = squares &females = circlesFilled circles/squares represent trait seen in personHalf colored circles/squares represent carriers (heterozygous for the trait and it is not seen)

Empty circles or squares do not have the trait nor carriers it.

A. Using PedigreesA pedigree is a useful tool for a geneticist.

Geneticist need to understand:Who had a trait through several generations.If a trait is recessive, dominant, sex-linked or has some other pattern of inheritance.How a trait is inherited – to predict the probability that a baby will born with a specific trait.

Pedigrees also are import in breeding animals or plants

Use for source of food (increase their yield and nutritional value.For desired physical and ability traits