Inheritance

Define the following terms:

Gene

Allele

**Gene**: A gene is heritable factor (segment of DNA) that controls for a trait or characteristic. (E.g. eye colour)

**Allele**: An allele is a variant form of a gene that occupies a specific position, or locus, on a chromosome. Alleles can exist in different forms and can result in variations in traits. For example, the gene for eye color may have alleles for blue, brown, or green eyes. Individuals inherit two alleles for each gene, one from each parent.

What is the difference between a genotype and a phenotype? Provide an example.

A genotype refers to the genetic makeup of an organism, while a phenotype refers to the observable traits resulting from the interaction between the genotype and the environment.

For example, one person’s genotype for eye color may be a combination of allels for brown and blue eyes. However, the brown allele is dominant and the blue allele is recessive. So their phenotype - the actual eye color - would be brown.

Define the following terms:

Dominant

Recessive

**Dominant:** In genetics, the term "dominant" refers to an allele that is expressed or observed in the phenotype when present in either one or two copies. It masks the effect of a recessive allele when both are present.

**Recessive:** In genetics, the term "recessive" refers to an allele that is only expressed or observed in the phenotype when present in two copies. It is masked by a dominant allele when both are present.

Define the following terms:

Homozygous dominant

Homozygous recessive

Hetereozygous

* **Homozygous dominant**: It refers to an individual having two identical dominant alleles for a particular trait. For example, if an organism has two dominant alleles for eye color (e.g., BB), it is homozygous dominant for that trait.
* **Homozygous recessive**: It refers to an individual having two identical recessive alleles for a particular trait. For example, if an organism has two recessive alleles for eye color (e.g., bb), it is homozygous recessive for that trait.
* **Heterozygous**: It refers to an individual having two different alleles for a particular trait. For example, if an organism has one dominant allele and one recessive allele for eye color (e.g., Bb), it is heterozygous for that trait.

What is a monohybrid cross?

A monohybrid cross is a genetic cross between two individuals that differ in only one trait. It involves the inheritance of a single gene with two different alleles.

Draw a Punnett square of a monohybrid cross between two plants heterozygous for two alleles: Y (yellow) and y (green). The Y allele is dominant.

State the **genotype** and **phenotype** ratios of the first filial generation (their offspring).

Genotype ratios: 25% YY 50% Yy 25% yy = **1:2:1**

Phenotype ratios: 75% Yellow 25% Green = **3:1**

What is Mendel’s first law of inheritance?

Mendel's first law of inheritance, also known as the law of segregation, states that during the formation of gametes (sex cells), the two alleles for a trait separate or segregate from each other, so that each gamete carries only one allele for each trait.

In other words, all individuals have two copies of each factor. These copies segregate (separate) randomly during gamete formation, and each gamete receives one copy of every factor.

What is true breeding?

A kind of breeding in which the **parents with a particular phenotype** produce **offspring only with the same phenotype**.

This means that the parents are **homozygous** for the trait.

Curly hair (C) is dominant over straight hair (c) in humans. Is it possible for a curly-haired man to produce curly-haired children if his wife has straight hair? Explain using Punnett squares.

Yes, it is possible for a curly-haired man to produce curly-haired children if his wife has straight hair.

**If the man is homozygous dominant** (CC) for curly hair and the woman is homozygous recessive (cc) for straight hair, their offspring will all be heterozygous (Cc) for curly hair. In this case, the dominant allele (C) will be expressed, resulting in curly hair in all the children.

Using a Punnett square, the possible genotypes of the offspring would be:

* 100% chance of Cc (curly hair)

**If the man is heterozygous (Cc)** for curly hair and the woman is homozygous recessive (cc) for straight hair, **half** of their offspring will be heterozygous (Cc) and **half** will be homozygous recessive (cc) for straight hair.

Using a Punnett square, the possible genotypes of the offspring would be:

* 50% chance of Cc (curly hair)
* 50% chance of cc (straight hair)

So in both cases, he can have children with curly hair.

Short hair (H) is dominant over long hair (h) in cats. If a homozygous dominant female mates with a homozygous recessive male, give the phenotype ratio of the **second filial generation (F2).**

The second filial generation (F2) would be 75% short hair and 25% long hair. The phenotype ratio would be **3:1**

What is a test cross?

A test cross is a breeding experiment used to determine the genotype of an individual with a dominant phenotype. Since you don’t know if this individual is **homozygous dominant** or **heterozygous**, you must cross the individual with a known **homozygous recessive** individual. The resulting phenotypes of the offspring can reveal the genotype of the individual being tested.

In some cases, you can determine the genotype of an organism by examining the phenotype alone.  In the case of Mendel’s pea plants, you know that round seeds (R) are dominant over wrinkled seeds (r).


1. Identify the genotypes for seed shape that you can determine by inspection alone. Explain.
2. How could you determine the genotypes that you cannot determine by inspection?
3. With what would you cross each of your unknowns?
4. Use a Punnett square to show the results for **test crosses** performed on all unidentified genotypes for seed shape.  Explain how each test cross can show which genotype you had in each case.

1. The genotype for seed shape that can be determined by inspection alone is **homozygous recessive** (rr). You can’t identify the genotype of round seeds because round (R) is dominant over wrinkled (r), so round seeds could be homozygous dominant (RR) or heterozygous (Rr).
2. To determine the genotypes that cannot be determined by inspection, you would need to perform a test cross. with a known homozygous recessive genotype. By observing the phenotypes of the offspring, you can determine the unknown genotype.
3. For each of the unknown genotypes, you would cross them with a known homozygous recessive genotype (rr).
4. **Draw the two possible Punnet squares for the test cross**.


1. If the test cross results show 100% Rr genotype (100% round) offspring, then the unknown is **homozygous dominant (RR).**
2. If the test cross results show a 1:1 genotype ratio (50% Rr, 50% rr) and a 1:1 phenotype ratio (50% wrinkled, 50% round), then the unknown is **heterozygous (Rr).**

What is Mendel’s second law of inheritance?

Mendel's second law of inheritance, also known as the law of independent assortment, states that the alleles for different traits segregate independently of one another during the formation of gametes. This means that the inheritance of one trait does not influence the inheritance of another trait.

The two alleles for one gene segregate (assort) independently of the alleles for other genes during gamete formation.

What is a dihybrid cross?

A dihybrid cross is a genetic cross between two individuals that differ in two traits or have two sets of alleles for two different genes.

In pea plants, the allele for round seeds (R) is dominant over the allele for wrinkled seeds (r). The allele for yellow seeds (Y) is dominant over the allele for green seeds (y).

Draw a **dihybrid cross** between two plants **heterozygous for both traits**.

State the **phenotype ratios** of the first filial generation (F1).

Punnet Square shown in image

Phenotype ratios = 9:3:3:1

9 Yellow, round (YYRR, YyRR, YYRr, YyRr)

3 Green, round (yyRR, yyRr)

3 Yellow, wrinkled (YYrr, Yyrr)

1 Green, wrinkled (yyrr)

In flies, the allele for a red eyes (G) is dominant over the allele for brown eyes (g). The allele for normal wings (E) is dominant over the allele for vestigial wings (e).

Draw a **dihybrid cross** between two flies **heterozygous for both traits**.

State the **phenotype ratios** of the first filial generation (F1).

Punnet Square shown in image

Phenotype ratios = 9:3:3:1

9 Red eyes, normal wings (GGEE, GgEE, GGEe, GgEe)

3 Brown eyes, normal wings (ggEe, ggEE)

3 Red eyes, vestigial wings (GGee, Ggee)

1 Brown eyes, vestigial wings (ggee)

What is incomplete dominance? How do we show incomplete dominant alleles in Punnet squares?

Incomplete dominance is a form of inheritance where neither of the two alleles is completely dominant over the other. Instead, they blend together to create an third, new intermediate phenotype in **heterozygous** offspring. For example, in snapdragons, the red and white alleles blend to produce pink flowers.

Both alleles are expressed in the heterozygote both take a **CAPITAL CASE** letter. An **index letter** identifies the allele. See image for examples.

What is codominance? Explain using an example.

Codominance is an inheritance pattern where both alleles in a heterozygous individual are fully expressed, without blending or dominance.

In the case of blood types, there are 3 alleles. 2 are dominant (A, B) and one is recessive (O). If an individual is heterozygous for A and B alleles, their phenotype becomes an **AB blood type**. In this case, both A and B antigens are present on the red blood cells. No blending occurs - they are both expressed fully.

The notation for codominant alleles uses an index letter, just like incomplete dominance. See image for examples.

What is the difference between incomplete dominance and codominance?

Incomplete dominance occurs when the heterozygous phenotype is an intermediate blend of the two homozygous phenotypes. For example, in snapdragons, a red flower (RR) crossed with a white flower (WW) produces pink flowers (RW).

Codominance, on the other hand, occurs when both alleles are fully expressed in the heterozygous phenotype. This means that both traits are visible simultaneously without blending. An example of codominance is the ABO blood group system, where individuals with AB blood type express both A and B antigens.

In summary, incomplete dominance results in an intermediate phenotype, while codominance results in the simultaneous expression of both alleles in the heterozygous phenotype.

If a male who is heterozygous for blood type A mates with a type AB female, what are the possible phenotypes and genotypes of their offspring?

Draw a Punnett square.

The possible phenotypes of their offspring would be blood types A, B, and AB. The possible genotypes would be:


1. 50% I^A I^A or I^A i (blood type **A**)
2. 25% I^A I^B (blood type **AB**)
3. 25% I^B i (blood type **B**)

Define polygenic inheritance. Give an example.

**Polygenic inheritance** refers to the inheritance of a trait that is controlled by multiple genes, each contributing a small effect to the phenotype. In this type of inheritance, the phenotype shows a continuous range of variation rather than distinct categories. An example of polygenic inheritance is human height, which is influenced by the combined effects of multiple genes.

What does “order of dominance” mean in polygenic inheritance? Give an example.

The "order of dominance" in polygenic inheritance refers to the hierarchy of gene expression where certain alleles have a greater influence on the phenotype than others. It determines the extent to which each allele contributes to the overall trait.

For example: In rabbits, the gene that controls coat colour in rabbits has 4 alleles: agouti (C), chinchilla (c^ch, Himalayan (c^h), and albino (c). \n \n The order of dominance is: \n \n C > c^ch > c^h > c (the symbol “>” means “is dominant to”)

An inheritance pattern for **cattle breeds** is as follows:

S (dutch belt) > s^h (hereford) > s^c (solid) > s (holstein)


1. A cow that is a heterozygous Dutch Belt (with a hereford allele) is crossed with a bull that is a holstein. What are the possible genotypes and phenotypes of their offspring (F1)?
2. If one of each type of offspring from the F1 generation was crossed, what would the possible genotypes and phenotypes of the F2 generation be?

SEE IMAGE FOR PUNNET SQUARES


1. F1 offspring


1. Genotypes = 1:1


1. 50% Ss (heterozygous Dutch belt with solid allele)
2. 50% s^hs (heterozygous hereford with holstein allele)
2. Phenotypes = 1:1


1. 50% Dutch Belt, 50% Hereford

\
2. F2 generation


1. Genotypes = 1:1:1:1


1. 25% Ss^h
2. 25% Ss
3. 25% s^h s
4. 25% ss
2. Phenotypes = 2:1:1


1. 50% Dutch Belt (Ss^h, Ss)
2. 25% Hereford (s^h s)
3. 25% Holstein (ss)

What is pleiotropy? Give an example of pleiotropy in humans (sickle cell anemia).

Pleiotropy refers to a genetic phenomenon where a single gene affects multiple traits or characteristics in an organism. An example of pleiotropy in humans is sickle cell anemia. The mutation in the HBB gene responsible for sickle cell anemia not only affects the shape of red blood cells but also influences other traits such as increased resistance to malaria.

Define epistasis. Give an example.

**Epistasis** is a genetic phenomenon where the expression of one gene masks or modifies the expression of another gene.

For example, coat color in mice. (SEE IMAGE)

Epistasis occurs in the interaction between the genes responsible for pigment production (e.g., gene A) and the gene responsible for pigment deposition (e.g., gene B). If gene A is not functional, it prevents the production of pigment regardless of the presence of a functional gene B, resulting in an albino phenotype. This demonstrates how the presence or absence of one gene can influence the expression of another gene in a non-additive manner.

Define sex-linked inheritance. Give an example of a sex-linked condition in humans.

Sex-linked inheritance refers to the inheritance of genes located on the sex chromosomes (X and Y). In humans, sex-linked conditions are typically associated with genes on the X chromosome.

One example of a sex-linked condition is color blindness, where the gene responsible for color vision is located on the X chromosome. See image for more details.

Sometimes inheritance does not follow the ratios that Mendel predicted using Punnett squares. Why does this occur?

This occurs due to **linkage**.

Linkage refers to the phenomenon where genes that are located close to each other on the same chromosome tend to be inherited together more frequently than predicted by Mendel's laws. This occurs because the closer two genes are on a chromosome, the less likely they are to undergo independent assortment during meiosis. As a result, the expected ratios of offspring predicted by Punnett squares may not be observed.

See image for an example.

How can linkage be **disrupted**? What does this lead to?

Linkage can be disrupted by genetic recombination, which is the exchange of genetic material between homologous chromosomes during meiosis. See image for an example.

Genetic recombination enables the production of offspring with new combinations of traits inherited from two parents.

Genetic recombination can result from independent assortment of genes located on non-homologous chromosomes or from crossing over of genes located on homologous chromosomes. Crossing over occurs in Prophase I of meiosis.

What is crossing over? How can we predict cross-over frequency?

Crossing over is the exchange of genetic material between homologous chromosomes during meiosis. It leads to genetic recombination and variation in offspring.

Cross-over frequency can be predicted using the formula: Cross-over frequency = Number of recombinant offspring / Total number of offspring.

Have you completed the **chromosome mapping questions** from Slides 127-137 of the Mendelian Genetics Powerpoint?

If no, go complete them!

If yes, good job 😊

What is pedigree analysis? State the pedigree inheritance patterns for the following conditions, and give examples of each.

Autosomal dominant

Autosomal recessive

X-linked recessive

Pedigree analysis is a method used to study the inheritance patterns of genetic traits within families.

**Autosomal dominant**: Trait is caused by a dominant allele located on an autosomal chromosome. It is expressed in individuals who have at least one copy of the dominant allele. Examples include Huntington's disease and Marfan syndrome. These diseases:


1. Show up in **EVERY** generation
2. Affect males and females equally

**Autosomal recessive**: In this pattern, the trait is caused by a recessive allele located on an autosomal chromosome. It is expressed in individuals who have two copies of the recessive allele. Examples include cystic fibrosis and sickle cell anemia. These diseases:


1. **Skip generations**
2. Affects males and females equally

**X-linked recessive**: Trait is caused by a recessive allele located on the X chromosome. It is **more commonly expressed in males** since they have only one X chromosome. Examples include color blindness and hemophilia.

The diagram shows a pedigree of cystic fibrosis, in which the black colour indicates the presence of cystic fibrosis.

What is the probability that the individual labelled X is a carrier of cystic fibrosis?

50% chance → **0.5 Probability**

What type of inheritance is shown in this pedigree chart?

X-linked recessive

What proves that the inheritance of the condition shown in this pedigree chart is autosomal recessive and not autosomal dominant?

Two unaffected parents have a child that is affected.

According to the pedigree shown, which pattern of inheritance is indicated?

Autosomal dominant

What type of inheritance is shown in this pedigree chart?

X-linked recessive

The pedigree chart below shows the blood types of three members of a family.

Which of the following could be the blood types of **individuals 1 and 2**?


1. A, AB
2. AB, B
3. O, B
4. B, A

4. B, A

What evidence is given in the pedigree chart to prove that the condition is caused by a dominant allele?

Two affected parents have an unaffected child.