Genetics Study Guide

Question 1

Who is the father of modern genetics? What did he study?

Answer 1

Gregor Mendel. Mendel studied inheritance patterns in pea plants. He discovered the basic principles of heredity, including the concepts of dominant and recessive traits, and formulated the Law of Segregation and Law of Independent Assortment, which form the foundation of modern genetics.

Question 2

Genotype

Answer 2

The genetic makeup of an organism, representing the combination of alleles it inherits from its parents (e.g., AA, Aa, aa) and describing the alleles it carries for a given trait.

Question 3

Allele

Answer 3

Different versions of a gene that can exist at a specific locus (location on a chromosome). For example, a gene for flower color may have an allele for red (R) or white (r).

Question 4

Gene

Answer 4

A segment of DNA that encodes information for a particular trait. Each gene can have different versions (alleles).

Question 5

Homozygous

Answer 5

An organism that has two identical alleles for a particular gene (e.g., AA or aa).

Question 6

Heterozygous

Answer 6

An organism that has two different alleles for a particular gene (e.g., Aa).

Question 7

Hybrid

Answer 7

The offspring resulting from the cross of two different genetic traits or different species. In genetics, it refers to the offspring of parents with different alleles for a particular trait (e.g., hybrid pea plant).

Question 8

Phenotype

Answer 8

The physical expression or observable traits of an organism, determined by its genotype and environmental factors (e.g., purple flowers or tall height).

Question 9

Law of Segregation

Answer 9

Mendel’s principle that states during the formation of gametes (egg and sperm), the two alleles for a gene segregate (separate), so each gamete carries only one allele for each gene.

Question 10

Law of Independent Assortment

Answer 10

Mendel’s principle that states genes for different traits assort independently of each other during gamete formation, provided the genes are located on different chromosomes.

Question 11

P Generation

Answer 11

The parental generation in a genetic cross.

Question 12

F1 Generation

Answer 12

The first filial generation, offspring of the P generation.

Question 13

F2 Generation

Answer 13

The second filial generation, offspring of the F1 generation, typically used in Mendelian experiments.

Question 14

Polygenic Inheritance

Answer 14

A type of inheritance where multiple genes contribute to a single trait (e.g., skin color, height). This results in a wide range of phenotypes.

Question 15

Test Cross

Answer 15

A breeding experiment used to determine the genotype of an organism with a dominant phenotype. The organism in question is crossed with a homozygous recessive individual, and the offspring’s phenotypes are analyzed to infer the unknown genotype.

Question 16

Example of Homozygous Dominant

Answer 16

AA
(Both alleles are dominant for a trait, such as the gene for purple flower color in pea plants.)

Question 17

Example of Heterozygous

Answer 17

Aa
(One dominant allele and one recessive allele. For example, in pea plants, a hybrid plant with one allele for purple flowers and one for white flowers.)

Question 18

Example of Homozygous Recessive

Answer 18

aa
(Both alleles are recessive for a trait, such as the gene for white flower color in pea plants, where two recessive alleles are needed for the white phenotype.)

Question 19

How many alleles are donated from each parent to their offspring for a particular trait?

Answer 19

Each parent will donate one allele to their offspring, resulting in possible genotypes like AA, Aa, or aa for the offspring.

Question 20

A Punnett square shows the actual or probable results from a genetic cross?

Answer 20

A Punnett square shows the probable results from a genetic cross.

Question 21

If you cross homozygous dominant yellow seeds with homozygous recessive green seeds all of your offspring will be what color and why?

Answer 21

Yellow:
The homozygous dominant yellow seeds (YY) have two dominant alleles for yellow color.

The homozygous recessive green seeds (yy) have two recessive alleles for green color.

When you perform the cross, each parent will donate one allele:
The YY parent will donate a Y allele (since both alleles are dominant).
The yy parent will donate a y allele (since both alleles are recessive).

The resulting offspring will all have the genotype Yy, meaning they have one dominant yellow allele (Y) and one recessive green allele (y).

Since yellow is dominant over green, the Y allele will mask the effect of the y allele, and all of the offspring will have yellow seeds.

Question 22

The allele for yellow seeds is dominant over green seeds. If you cross a yellow and green seed plant and you get 48 yellow and 53 green, what are the genotypes of the parents?

Answer 22

A 1:1 ratio suggests that one parent is heterozygous (Yy) and the other parent is homozygous recessive (yy).

Question 23

In flies red eyes are recessive to the wild type. If you have two wild type flies and they produce a fly with red eyes, what are the genotypes of the parents?

Answer 23

The genotypes of the parents must both be Rr (heterozygous wild type).

Red eyes (r) are recessive.
Wild type eyes (R) are dominant over red eyes.
For a fly to have red eyes (rr), it must inherit two recessive alleles (one from each parent).

Question 24

In Hamsters Black fur is dominant to brown. If you cross a brown hamster with a black hamster, who had a parent that was brown, what are the chances of having a black hamster?

Answer 24

There is a 50% chance (2 out of 4) that the offspring will inherit the Bb genotype (black fur).
There is a 50% chance (2 out of 4) that the offspring will inherit the bb genotype (brown fur).

A brown hamster must have the genotype bb (homozygous recessive), as it can only pass on the b allele.
The black hamster with a brown parent must be heterozygous (Bb), because the black hamster inherited the b allele from the brown parent.

Question 25

What is incomplete dominance? How many phenotypes can be expressed?

Answer 25

Incomplete dominance is a genetic phenomenon in which neither allele for a gene is completely dominant over the other. As a result, the heterozygous phenotype is a blend or intermediate between the two homozygous phenotypes.

In incomplete dominance, there are usually three distinct phenotypes:
1. Homozygous dominant phenotype (e.g., red flowers)
2. Homozygous recessive phenotype (e.g., white flowers)
3. Heterozygous phenotype (e.g., pink flowers)

Question 26

Show a cross between a red flower and a white flower. What are the genotypic & phenotypic results?

Answer 26

Genotypic Results:
100% RW (heterozygous) - These offspring will all have pink flowers.

Phenotypic Results:
100% Pink Flowers (since the heterozygous RW phenotype shows as pink, the intermediate color between red and white).

Question 27

Show a cross between two pink flowers. What are the genotypic & phenotypic results?

Answer 27

Genotypic Results
25% RR (Red flowers)
50% RW (Pink flowers)
25% WW (White flowers)

Phenotypic Results
25% Red flowers (from RR)
50% Pink flowers (from RW)
25% White flowers (from WW

Question 28

What is codominance?

Answer 28

A type of genetic inheritance in which both alleles for a gene are fully expressed in the heterozygous organism, resulting in a phenotype that displays both traits simultaneously, without blending.

Question 29

Cross a person that has type A blood (genotype Ao) and a person with type O blood. What are the possible genotypes & phenotypes in the offspring?

Answer 29

Person with Type A blood (genotype Ao): This person has one A allele (Iᵃ) and one O allele (i). The genotype is Iᵃi.

Person with Type O blood (genotype ii): Type O blood individuals are homozygous recessive, so their genotype is ii.

Genotypic Results:
50% Iᵃi (Type A blood)
50% ii (Type O blood)

Phenotypic Results:
50% Type A blood
50% Type O blood

Question 30

A person has type B blood and marries a person with type A blood. What are their
genotypes if they have a child with type O blood?

Answer 30

Type B blood parent: Since the child has type O blood, this parent must be heterozygous (Iᵇi). If this parent were homozygous (IᵇIᵇ), they could only pass on the Iᵇ allele, and the child would not be able to have type O blood.

Type A blood parent: Similarly, since the child has type O blood, this parent must be heterozygous (Iᵃi). If the parent were homozygous (IᵃIᵃ), they would only pass on the Iᵃ allele, and the child could not have type O blood.

Question 31

Define sex linked traits

Answer 31

Traits that are controlled by genes located on the sex chromosomes (X or Y). Most sex-linked traits are X-linked, meaning they are carried on the X chromosome. Since females have two X chromosomes (XX) and males have one X and one Y chromosome (XY), sex-linked traits often show different inheritance patterns in males and females.

Question 32

Why are men more susceptible to X linked genetic disorders? How can a female end up with an X-linked genetic disorder?

Answer 32

Men are more susceptible to X-linked genetic disorders because they have only one X chromosome (XY). If the X chromosome carries a recessive allele for a genetic disorder, there is no second X chromosome to potentially “mask” the effect. As a result, men will express the disorder if they inherit the defective X chromosome from their mother.

Females have two X chromosomes (XX), so for a female to express an X-linked recessive disorder, she must inherit two copies of the defective allele—one from each parent. If she inherits a normal X chromosome from one parent and a defective X chromosome from the other parent, she will be a carrier (heterozygous) but will not express the disorder. However, if both X chromosomes carry the defective allele, the female will express the disorder, just like males do.

Question 33

What is Hemophilia and how is it inherited?

Answer 33

A genetic disorder where the blood doesn’t clot properly, leading to excessive bleeding even from minor injuries.

Hemophilia is most commonly inherited in an X-linked recessive manner, meaning males (XY) are more likely to be affected because they have only one X chromosome. Females (XX) must inherit two copies of the defective gene (one from each parent) to be affected, though they are often carriers if they have one normal X and one mutated X.

Question 34

If a colorblind man marries a woman who is a carrier for colorblindness, what is the chances their daughters could be colorblind? Their sons?

Answer 34

Colorblind man: Since colorblindness is X-linked recessive, the man must have the genotype XᴄY, where Xᴄ represents the X chromosome carrying the colorblind allele, and Y represents the Y chromosome.
Carrier woman: A woman who is a carrier for colorblindness has one normal X chromosome and one X chromosome with the colorblind allele. Her genotype is XᴺXᴄ, where Xᴺ is the normal X and Xᴄ is the X carrying the colorblind allele.

Daughters:
50% will be XᴄXᴺ (normal vision, but carrier).
50% will be XᴄXᴄ (colorblind).
Sons:
50% will be XᴄY (colorblind).
50% will be XᴺY (normal vision).

Question 35

If the colorblind woman marries a man that is not colorblind, what are the chances their daughters will be colorblind? Their sons?

Answer 35

Colorblind woman: Since colorblindness is X-linked recessive, the woman must have two copies of the defective X chromosome. Therefore, her genotype is XᴄXᴄ, where Xᴄ is the X chromosome carrying the colorblind allele.
Non-colorblind man: A non-colorblind man has one normal X chromosome and one Y chromosome. His genotype is XᴺY, where Xᴺ is the normal X chromosome.

Daughters:
100% of the daughters will inherit Xᴄ from their mother and Xᴺ from their father, resulting in the genotype XᴄXᴺ (carrier of colorblindness). They will not be colorblind, but will carry the gene.
Sons:
100% of the sons will inherit the Y chromosome from their father and one Xᴄ from their mother, resulting in the genotype XᴄY (colorblind). All sons will be colorblind.

Question 36

Who determines the sex of the baby in humans?

Answer 36

In humans, the father determines the sex of the baby. This is because the father contributes either an X or a Y chromosome, while the mother always contributes an X chromosome.

Question 37

What are the sex chromosomes of a female? Male?

Answer 37

Female: The sex chromosomes are XX. A female has two X chromosomes, one inherited from each parent.

Male: The sex chromosomes are XY. A male has one X chromosome (inherited from the mother) and one Y chromosome (inherited from the father).

Question 38

Fragile X Syndrome

Answer 38

Caused by a mutation on the X chromosome, which leads to an abnormal repetition of the CGG sequence.

Symptoms:
1. Intellectual disability
2. Social anxiety or difficulty with social interactions
3. Hyperactivity and impulsive behavior
4. Speech and language delays
5. Sensory sensitivities
6. Long face, large ears, and a prominent jaw (more noticeable in males)
7. Increased risk of autism spectrum disorders in affected males

Question 39

Turner syndrome

Answer 39

Occurs when a female is born with only one X chromosome instead of two. The second X chromosome is either missing or incomplete.

Symptoms:
1. Short stature (average adult height is about 4’8” to 5’0”)
2. Infertility (due to underdeveloped ovaries and lack of ovarian function)
3. Heart defects
4. Low-set ears and webbed neck
5. Normal intelligence, though some may have learning disabilities, especially in spatial reasoning and mathematics
6. Delayed puberty or lack of menstrual periods without hormone replacement
7. Hearing problems or ear infections may be common

Question 40

Cri-du-Chat Syndrome

Answer 40

Caused by a deletion of a portion of the 5th chromosome

Symptoms:
1. High-pitched crying
2. Intellectual disability
3. Delayed speech and motor skills development
4. Distinctive facial features: Round face, wide-set eyes, small jaw, and low-set ears
5. Growth delays (short stature)
6. Heart defects, especially ventricular septal defects
7. Hypotonia (low muscle tone)