Download Chapter 5.3 – Human Genetics (Part I)

NAME: _____________________________________
PERIOD: ________
Chapter 5.3 – Human Genetics (Part I)
Other Patterns of Inheritance: pages 354 – 356
Mendel’s work in genetics provided the foundation for our knowledge of genetics.

Not all traits are dominant or recessive, some traits appear to blend.

Some of the traits that “blend” in humans are skin color, eye color, hair texture and blood type.
The traits that Mendel studied were single-gene traits.

All 7 traits were located on nonhomologous chromosomes and were, therefore, inherited
independent of the other.

Most of the genes that affect the physical appearance of humans are located on autosomes.
What is an autosome? Chromosome that does not determine sex (44/22 sets).
Not all traits follow such a simple inheritance pattern.

Many traits are not completely dominant or recessive.

Many traits are controlled by more than one gene at different locations on the chromosome.

This is called non-Mendelian inheritance.
Non-Mendelian Inheritance Patterns:

Incomplete Dominance –
o
One allele does not completely mask the action of the other allele.
o
Neither allele is completely dominant.
o
Example – Snapdragons (a type of flower)

Red flowers are not completely dominant over White flowers – this results in a
“blending” of the two phenotypes


Pure red x Pure white = Pink flower (heterozygous)

What ratio of genotypes and phenotypes would be expected in the F2 generation?
Codominance –
o
Both alleles are expressed equally.
o
Examples –

cattle and horse coat color - Red hair + White hair = Roan (red and white) color
(heterozygous)

black feathered chicken + white feathered chicken = black and white feathered
chicken (heterozygous)
NAME: _____________________________________
 Multiple Alleles –
PERIOD: ________
o
Some genes have more than two alleles that affect the trait.
o
For example, in humans blood type is affected by three different alleles: A, B, and O.
o
There is only one gene on a chromosome that controls blood type, but there are three
different forms of the gene.

Blood Group Phenotypes
Possible Genotypes
A
AA,AO
B
BB or BO
AB
AB
O
OO
o
If a person has type A blood, what possible genotypes could they have? AA, AO
o
What is the only possible genotype for a person with type O blood? O
Polygenic Traits –
o
This type of inheritance involves multiple genes for a single trait.
o
Eye color and skin color are polygenic traits.
o
In humans, skin color is a single trait but over four different genes are involved. There
are many different combinations.
Skin Phenotypes
Possible Genotypes
White (albino)
aabb
LIGHT
Aabb or aaBb
Medium
AAbb, AaBb, aaBB
Dark
AaBB, AABb
BLACK
AABB
o
The more dominant alleles present, the darker the skin tone.
o
A person with white (albino) skin has all recessive alleles in their genotype, while a person
with darker skin has all dominant alleles in their genotype.
Environmental Influences:

The environment influences the expression of genes.

Height is influenced by nutrtition as well as the release of growth hormones.

Plant height and leaf development is influenced by the amount of soil and water available.
NAME: _____________________________________
PERIOD: ________
 How can identical twins have the same genes but not look the same? Environmental influences can
affect the appearance of identical twins – Nutrition and Horomones
Mutations Cause Some Genetic Disorders: pages 356 – 358
A genetic disorder is a disease caused by an abnormality in the DNA .

Some genetic disorders are caused a mutation in a single gene.

These disorders are inherited in Mendelian patterns, such as autosomal recessive, autosomal
dominant or sex-linked patterns.
Autosomal Recessive Disorders:

Most genetic disorders in humans are caused by recessive alleles on autosomes.

In order to inherit this trait, a person must inherit two copies of the recessive allele.

Examples include… sickle-cell disease, cystic fibrosis and phenylketonuria.

Sickle-cell disease –
o
Affects the formation of red blood cells.
o
There is a mutation in the hemoglobin protein that causes RBC’s to flatten with an
irregular shape.
o
How does sickle-cell disease affect a patient with this disorder? Transporting oxygen
o
Normal hemoglobin - H, dominant allele - Sickle-cell - h, recessive allele
o
Individuals who are heterozygous (Hh) for sickle-cell are more resistant to malaria, a
disease transmitted through mosquitoes .
o
Why is sickle-cell disease common in subtropical regions, such as Africa, India and parts of
the Middle East? Malaria is more prevalent in these regions.
Chapter 5.3 – Human Genetics (Part II)
Autosomal Dominant Disorders

Less common than autosomal recessive disorders, because the presence of a single allele may be
lethal (deadly).

If just one parent is heterozygous for the trait, there is a 50% chance of the offspring
inheriting the trait. If both parents are heterozygous, there is a 75% chance.

Examples include… Huntington’s disease and Marfan syndrome.

Huntington’s disease –
NAME: _____________________________________
PERIOD: ________
o causes damage to nerve cells, eventually causing them to waste away
o
What effects does this nerve damage have on a person? Brain wastes away, emotionally
disturbed, loss of intellect
o
Do not typically show symptoms until later in life, around 20s to 30s , and individuals
usually only live 15-20 years after symptoms start.

Marfan syndrome –
o
caused by a mutation that affects collagen formation; collagen is a connective tissue found
in tendons, ligaments, cartilage, blood vessels and heart valves
o
What effects does this syndrome have on a patient? Heart Problems, Scoliosis, Eye
Problems
o
What are some other symptoms of Marfan syndrome? Long arms and long legs (irregular)
Answer the following discussion questions based on what you have read:
1. How is incomplete dominance different from codominance? Incomplete dominance blends the
traits, whereas codominance expresses both traits equally.
2. Why does a polygenic trait have a wide range of phenotypes in the population? Polygenic traits
have a wide range because of all the possible different allele combinations.
3. Show the results of a cross between an individual that is homozygous for A type blood and an
individual that is heterozygous for B type blood. List the probably genotypic and phenotypic
ratios of the offspring. AA x BO = AB, AO
4. Show the results of a cross between two individuals that have type AB blood. List the probably
genotypic and phenotypic ratios of the offspring. AB x AB 1AA:2AB;1BB
5. Show the results of a cross between an individual heterozygous for Huntington’s disease and an
individual without the disease. What is the percent chance that the offspring will inherit
Huntington’s disease? Hh x HH: 0% will have it; 50% will carry the recessive trait
6. Show the results of a cross between two individuals heterozygous for sickle-cell disease. What is
the percent chance that the offspring will not have sickle-cell disease? Ss x Ss; 75% will not
Sex Determination: pages 360 – 361

Females have two X chromosomes (XX) and males have an X and a Y chromosome (XY).

The Y chromosome is shorter than the X and contains genes that code for the development of
the male .
NAME: _____________________________________
PERIOD: ________
 If the Y chromosome is absent or missing the sex-determining region, the fetus will develop into
a female.

Fill in the following Punett Square. Label the male parent. Label the female parent.
X
X
XX
Y
XY
X
XX
XY

What is the percent chance that the parents will have a male baby? 50%

What is the percent chance that the parents will have a female baby? 50%

If two parents have a male baby, does this mean that they are less likely to have a male baby the
next time? Explain. No! Regardless, the chances for having a girl or a boy will always be
50%.


What are Sex-linked genes? genes located on the sex chromosomes

How is sex-linked inheritance different from autosomal inheritance? They are found on either
the X or Y chromosomes.

Examples of sex-linked traits found on the X chromosome are:
Hemophilia and Colorblindness
Karyotypes: page 366
Define karyotype – picture/map of all the chromosomes in a cell.
A normal human has 46 chromosomes:

44 are autosomes

2 are sex chromosomes

Male Karyotype: 46, XY - Female Karyotype: 46, XX

Which chromosome is the largest? X

Which chromosome is the smallest? Y

What is the sex of a person with the following karyotype?
a) MALE
b) FEMALE
NAME: _____________________________________

PERIOD: ________
Examine the following karyotype:
a) What is not normal about this karyotype?
Trisomy 21
b) Is this individual male or female? Female
c) What is the name of this condition? Down
Syndrome
d) This condition is a result of a nondisjunction
mutation. What does this mean? Chromosomes
did not pull apart during meiosis.