3A6 Non-Mendelian Genetics

Question 1

What is an inheritance pattern where both dominant genes are expressed in heterozygous individuals rather than blending?

Answer 1

Codominance

Question 2

What is a pattern of inheritance where heterozygous individuals will display a blend of traits?

Answer 2

Incomplete dominance

Question 3

What are sex-linked traits?

Answer 3

Traits found on the X chromosome.

Question 4

Why are males more likely to be affected by sex-linked conditions like color-blindness?

Answer 4

Males have only one X chromosome, so a single copy of the color-blind allele will determine their phenotype.

Question 5

What is the advantage of having two X chromosomes in females regarding sex-linked traits?

Answer 5

Disadvantageous recessive phenotypes can be masked in a heterozygote.

Question 6

What is a sex-limited trait and where are they usually found?

Answer 6

A trait that is only expressed in one of the sexes. Typically found on an autosome, though they are only expressed in one sex.

Question 7

What is true of genes that are said to be linked?

Answer 7

The genes are found on the same chromosome.

Question 8

What are multiple alleles?

Answer 8

Three or more different alleles of a gene.

Traits controlled by multiple alleles include:

• Blood type in humans
• Fur color in animals

Question 9

What happens when a person inherits one I^A allele and one I^B allele for blood type?

Answer 9

They have blood type AB.

When both I^A and I^B alleles are present, they are co-dominant, resulting in blood type AB.

Question 10

How is the i allele different from I^A and I^B alleles for blood type?

Answer 10

It is recessive towards both I^A and I^B alleles.

The i allele is recessive, meaning it is not expressed when I^A or I^B alleles are present.

Question 11

What are monogenic traits?

Answer 11

Traits that are controlled by a single gene and typically follow Mendelian inheritance patterns, such as cystic fibrosis or Huntington’s disease.

Question 12

What is a polygenic trait?

Answer 12

A trait in which the phenotype is controlled by more than one gene.

Examples of polygenic traits in humans:

• Eye color
• Skin color
• Height
• Hypertension
• Diabetes
• Schizophrenia
• Asthma
• Cancer

Question 13

What is polygenic inheritance?

Answer 13

It is an inheritance where one characteristic is controlled by two or more genes.

Question 14

Define the characteristics of polygenic traits.

Answer 14

• Quantitative
• Multifactorial
• Controlled by multiple genes or environment

Quantitative: Having continuous variation.

Question 15

What does it mean when polygenic traits are considered multifactorial?

Answer 15

Multiple genes and/or the environment influence the phenotype.

Question 16

How do polygenic traits contribute to natural selection?

Answer 16

They create a wide variety of phenotypes in a species, increasing the likelihood of a good fit for the environment.

Question 17

What makes polygenic traits different from Mendelian traits?

Answer 17

• Polygenic traits are controlled by multiple genes.
• Mendelian traits are inherited at a single gene with two alleles.

Question 18

What is co-dominant inheritance?

Answer 18

Both alleles are expressed together in the heterozygous organism.

Question 19

What is epistasis?

Answer 19

The phenomenon of one gene suppresses or interferes with the physical expression of another gene.

Polygenic traits that require the presence of an epistatic gene:

• Hair color
• Eye color
• Skin color

Question 20

What are the types of epistasis?

Answer 20

• Recessive epistasis
• Dominant epistasis

Question 21

When does dominant epistasis occur?

Answer 21

When there is complete dominance of one gene.

Question 22

What is recessive epistasis?

Answer 22

Occurs when recessive alleles on one gene interrupt or mask the expression of another gene.

Example: Albinism

Question 23

What are the causes of epistasis?

Answer 23

• Mutagens or physical and chemical influences which can cause a gene to mutate and be broken.
• Recombination during meiosis.
• Nondisjunction during meiosis.

Question 24

How can epistasis be a factor in disease?

Answer 24

It can worsen the severity of diseases, such as Alzheimer’s, by interacting with other genes to exacerbate the condition.

Question 25

What is **pleiotropy**?

Answer 25

One gene affecting **multiple traits**. ## Footnote Examples: the TBX5 gene codes for T-box 5 protein, affecting upper limbs and heart.

Question 26

What is the **difference** between pleiotropy and polygenic inheritance?

Answer 26

* **Pleiotropy**: one gene affects multiple traits. * **Polygenic inheritance:** many genes influence one trait.

Question 27

What is **antagonistic pleiotropy**?

Answer 27

When a trait that increases fitness in one area decreases fitness in another.

Question 28

What is **senescence**?

Answer 28

The deterioration associated with **aging**.

Question 29

How does antagonistic pleiotropy relate to senescence?

Answer 29

Traits caused by a gene that benefit early life but contribute to age-related decline.

Question 30

What is the **difference** between antagonistic pleiotropy and normal pleiotropy?

Answer 30

* **Antagonistic** pleiotropy involves traits with competing effects. * **Normal** pleiotropy involves a single gene affecting multiple traits.

Question 31

Provide examples of pleiotropic diseases.

Answer 31

* Sickle cell anemia * Phenylketonuria (PKU) ## Footnote Sickle cell anemia alters the shape of red blood cells making them sickle-shaped and unable to adequately transport oxygen. Phenylketonuria prevents the breakdown of phenylalanine into tyrosine.

Question 32

What is **cytoplasmic inheritance**?

Answer 32

Inheritance of genes in the cytoplasm of a cell not within the nucleus. ## Footnote Also known as extranuclear inheritance.

Question 33

What is the **distinct feature** of cytoplasmic inheritance?

Answer 33

It refers to traits **passed through genes in organelles** like mitochondria, which are inherited only from the **mother**.

Question 34

What is **mitochondrial inheritance**?

Answer 34

Inheritance of genes from mitochondrial DNA, mainly maternal.

Question 35

What is **mitochondrial DNA** also known as?

Answer 35

mtDNA

Question 36

What organelles contain their own copies of DNA similar to mitochondria?

Answer 36

**Chloroplasts** found in plant cells.

Question 37

How does the inheritance pattern of **mitochondria** and **chloroplasts** differ from nuclear DNA?

Answer 37

They follow the inheritance pattern of cytoplasmic inheritance, not Mendelian genetics.

Question 38

What are the **three types** of cytoplasmic inheritance?

Answer 38

* Vegetative segregation * Uniparental inheritance * Biparental inheritance

Question 39

What is **vegetative segregation**?

Answer 39

Random replication and separation of organelles in the cytoplasm during cell division, producing daughter cells with a random sampling of parental cytoplasmic organelles.

Question 40

When does **uniparental inheritance** occur?

Answer 40

When offspring inherit extranuclear genetic information from a single parent.

Question 41

When does **biparental inheritance** occur?

Answer 41

When extranuclear genetic information is inherited from both parents. ## Footnote Very rare and can only occur in some species.

Question 42

What is **epigenetics**?

Answer 42

The study of how **environmental factors** can influence heritable traits. ## Footnote It does not involve changes to the underlying DNA sequence.

Question 43

Why are humans considered **poor model organisms** for the study of genetics?

Answer 43

* Long generation times * Inability to make controlled crosses * Limited number of offspring * Restrictions on genetic manipulation

Question 44

What do human geneticists study **in the absence** of controlled crosses and genetic manipulation?

Answer 44

* Human genome in its natural context * Naturally occurring mutations * Family pedigrees

Question 45

What is a **pedigree**?

Answer 45

Charts of family histories that show **phenotypes** and **family relationships** of individuals.

Question 46

What can be determined by **analyzing pedigrees** in the study of human genetics?

Answer 46

* Whether a genetic condition is dominant or recessive. * Whether a condition is caused by a gene on the X chromosome or an autosome.

Question 47

How are **males** and **females** represented in a pedigree?

Answer 47

* Male represented by a **square** * Female represented by a **circle**

Question 48

What does an **open shape** in a pedigree represent?

Answer 48

Individual **does not** exhibit the trait of interest.

Question 49

What does a **filled-in shape** in a pedigree represent?

Answer 49

Individual **exhibits** the trait of interest.

Question 50

What is a proband in a pedigree?

Answer 50

The first affected family member seeking medical attention. ## Footnote Marked with an angled arrow labeled with a capital P pointing to the filled in shape.

Question 51

How are **family relationships** represented in a pedigree?

Answer 51

* **Marriage line** a horizontal line connecting partners. * **Vertical line** connects to children. * **Divorce** is represented by a break in the relationship line that connects the couple.

Question 52

How are **twins** represented in a pedigree?

Answer 52

By a **bifurcated vertical** line. ## Footnote Monozygotic twins are represented by connecting the split line with a horizontal line.

Question 53

How is **death** indicated in a pedigree?

Answer 53

By placing a **diagonal line over the symbol** of the deceased individual.

Question 54

How are miscarriages and abortions shown on a pedigree chart?

Answer 54

* **Miscarriage**: a small triangle shape attached with a shorter vertical line. * **Filled-in triangle** indicates the fetus was *affected*. * A **diagonal line over the triangle** if pregnancy was *terminated*.

Question 55

What is the **difference** between autosomal and sex-linked conditions?

Answer 55

* **Autosomal** conditions are caused by genes on chromosomes 1-22. * **Sex-linked** conditions are caused by genes on the sex chromosomes, typically the X.

Question 56

Why are complex diseases more challenging to study than simple autosomal conditions?

Answer 56

They have contributions from **multiple gene loci** and non-genetic factors, making them more difficult to understand.

Question 57

What method do human geneticists use to study complex diseases due to their multifactorial nature?

Answer 57

Population genetics

Question 58

How do geneticists try to minimize the impact of non-genetic factors when studying complex diseases?

Answer 58

By **matching non-genetic factors** between affected and unaffected individuals.

Question 59

What type of condition is caused by a **gene on an autosome**?

Answer 59

Autosomal condition ## Footnote Examples: * Huntington's disease * Polycystic kidney disease * Cystic Fibrosis

Question 60

What is the inheritance pattern for autosomal **dominant** traits?

Answer 60

An individual will develop the condition if they get **a copy of the gene** from either one of their parents.

Question 61

What is a **carrier** in genetics?

Answer 61

Someone who has one copy of the gene for a genetic condition but also has a normal allele that prevents them from developing the condition.

Question 62

How are carriers identified in a **pedigree for autosomal recessive traits**?

Answer 62

*If two unaffected parents have offspring that are affected*, then the pedigree is showing an autosomal recessive trait.

Question 63

Explain the difficulty in identifying autosomal recessive conditions in pedigrees.

Answer 63

Most of the recessive alleles remain hidden in unaffected carriers and only become apparent in a small number of family members, often separated by generations.

Question 64

How are carriers identified in a pedigree for **sex-linked traits**?

Answer 64

If there are *affected male children, unaffected female children, and unaffected parents*, then it is most likely a sex-linked trait and the **mother is a carrier**.

Question 65

Describe the **inheritance pattern** of sex-linked traits.

Answer 65

Sex-linked traits often follow a **crisscross** pattern of inheritance. ## Footnote Affected males pass a recessive sex-linked allele to their daughters, who become carriers of the trait. These carrier females then pass the affected allele to half of their sons and daughters.