Genetics and Heredity
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Genetics and Heredity Part II. Is the study of genetics and heredity really as simple as 2 alleles and a Punnett square?. Unfortunately, NO. Darn . . . The following concepts make genetics more interesting and also more complicated:

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Is the study of genetics and heredity really as simple as 2 alleles and a punnett square
Is the study of genetics and heredity really as simple as 2 alleles and a Punnett square?



Darn . . .

The following concepts make genetics more interesting and also more complicated:

- incomplete dominance

- multiple alleles

- polygenic inheritance

- environmental factors

- chromosome disorders

- sex-linked disorders

Incomplete dominance
Incomplete Dominance also more complicated:

Incomplete dominance occurs when the offspring of 2 homozygous parents show an intermediate phenotype. (#6)

+ also more complicated:


Red and white are equally dominant colors also more complicated:

first generation

Second generation: red and white reappear

curly also more complicated:



Type of Hair


Multiple alleles
Multiple Alleles also more complicated:

Many traits are controlled by multiple alleles. They produce 3 or more phenotypes.

Ex: also more complicated:blood types

3 alleles for blood

A, B, O


4 blood types:

A, AB, B, O

Polygenic inheritance
Polygenic Inheritance also more complicated:

Polygenic inheritance occurs when a group of genes work together to produce one trait. #9

As a result, many phenotypes are produced. also more complicated:

These 3 gene pairs work together to determine one trait.

Ex: also more complicated: hair color, eye color, skin tone

Environmental Factors also more complicated:

Your environment may help determine which traits are expressed.

Ex: also more complicated:Your diet, exposure to chemicals or sunlight, and temperature can affect genetics.

Chromosome disorders
Chromosome Disorders also more complicated:

Every species has a specific # of chromosomes.

Humans have 46 chromosomes or 23pairs.

Human Chromosomes also more complicated:

Pair #23 determines gender:

XX = girl

XY = boy.

Sometimes an error occurs during also more complicated:meiosis and a gamete (egg or sperm) has too many or too few chromosomes.

# of chromosomes ≠ 23

A zygote formed with one of these defective gametes usually dies
A also more complicated:zygote formed with one of these defective gametes usually dies.

If the zygote survives, the baby will be born with a disorder or disability.

Examples of chromosome disorders
Examples of also more complicated:Chromosome Disorders

Down’s Syndrome: 3 copies of chromosome #21; short stature, oversized heart, learning disabilities


Turner syndrome: also more complicated:only affects girls; are missing part or all of one X chromosome; neck webbing, short, infertile, severe heart and kidney problems (#13)

Autism: also more complicated:brain disorder; repetitive movements, delays in speech, learning, and socialization

Marfan’s Syndrome: affects connective tissue; thin body, narrow face, long limbs


This girl is missing part of her also more complicated:18th chromosome.

This boy is missing part of the 11thchromosome. (Jacobsen’s Syndrome) (#13)

Sex-Linked Disorders also more complicated:

Some conditions are passed only on the 23rd or the X or Y chromosome.

Since these are our sexcells, the disorders are sex-linked.

All eggs carry the also more complicated:X chromosome. The sperm determines the gender of a child.

Females: XX

Males: XY




It’s a BOY!!

It’s a GIRL!!

If a disorder is carried on the x chromosome both boys and girls can inherit it 12
If a disorder is carried on the also more complicated:X chromosome, both boys and girls can inherit it. (#12)

Males:XY Females:XX

If a disorder is carried on the Y chromosome, only boys can inherit it.

Punnett square for a sex-linked disorder that’s carried on the X chromosome.

Mother does not have the disorder, but carries and passes on the gene for it.

Mother (carrier)

Examples of sex linked disorders
Examples of Sex-Linked Disorders the X chromosome.




Duchenne Muscular Dystrophy the X chromosome.


Calico (cats’ fur)

Hemophilia the X chromosome.

This hemophilia patient received only one injection (shot).

The body reacted with severe bleeding under the skin. (bruising)


Pedigrees the X chromosome.

A pedigree is a chart used to trace genetic traits throughout a family’s history.

A pedigree is very useful when trying to find the the X chromosome.pattern of inheritance of a genetic disorder.

(colorblindness, baldness, hemophilia)