Unit 5 - Heredity

Question 1

What are the P, F1, and F2 generations?

Answer 1

P = parental generation; true-breeding parents in a genetic cross

F1 = first filial generation; offspring of P parents

F2 = second filial generation; offspring of F1 parents

Question 2

What is the law of independent assortment? Which phase of meiosis is it exhibited in?

Answer 2

DESCRIPTION

Each pair of alleles for different genes will sort independently during gamete formation

In other words, the allele a gamete receives for one gene (blue eyes) does not influence the allele received for another gene (sharp nose).

MEIOSIS PHASE

It is exhibited in metaphase I of meiosis because the maternal and paternal chromosomes of each homologous pair line up in random order at the metaphase plate

Question 3

Alleles

Answer 3

Definition: Alternative versions of genes with slightly different DNA sequences

Alleles account for variations in inherited characteristics among offspring

EX: Gene/characteristic = flower color and alleles = white and purple

Question 4

Dominant vs Recessive allele (normal case)

Answer 4

If the two alleles are different, the dominant allele will be fully expressed in the offspring and determine its phenotype while the recessive allele will have no noticeable effect

Question 5

What is the law of segregation? Which phase of meiosis is it exhibited in?

Answer 5

DESCRIPTION

The two alleles for a character segregate (separate) during gamete production and end up in different gametes.

An egg/sperm gets only the maternal allele or only the paternal allele which were both present in the somatic cell

If the parent has two of the same alleles from the mother and father (true-breeding), then the offspring will all get that version of the gene, but if the parent has two different alleles, each offspring has a 50% chance of getting one of the two alleles

MEIOSIS PHASE

It’s exhibited in anaphase I of meiosis because the separation of homologs means the separation of alleles

Question 6

Phenotype

Answer 6

An organism’s expressed physical traits (EX: purple color)

Question 7

Genotype

Answer 7

An organism’s genetic makeup (EX: PP)

Question 8

What are the rules of probability?

Answer 8

To find the chances of event 1 AND event 2 = MULTIPLY

To find the chances of event 1 OR event 2 = ADD

Question 9

What is complete dominance?

Answer 9

Heterozygous and homozygous dominant organisms are indistinguishable. Could occur when just one single dominant allele produces a sufficient amount of material/protein/substance to support the individual

Question 10

What is incomplete dominance? Give an example.

Answer 10

Neither allele is completely dominant, so F1 hybrids will have a phenotype somewhere between those of the two parental varieties

EX: red flowers + white flowers –> pink flowers (R:P:W = 1:2:1)

Question 11

What is codominance? Give two examples.

Answer 11

Two alleles are dominant and both are expressed on the phenotype of the offspring

EX: brown cow + white cow –> brown and white spotted cow
EX 2: Human blood types - alleles A and B are codominant; someone who has both alleles has blood type AB

Question 12

What are multiple alleles? Give an example.

Answer 12

Genes can exist in more than 2 allelic forms

EX: There are 3 possible alleles for human blood types (IA, IB, and i) but one person can receive any combination of 2 alleles

Question 13

What is pleiotropy? Give an example.

Answer 13

Property of a gene that causes it to affect more than one phenotypic character (opposite of polygenic inheritance)

EX: Sickle cell disease has multiple symptoms all caused by a single defective gene

Question 14

What is polygenic inheritance? Give an example.

Answer 14

Multiple genes have an additive effect on a single phenotype character (opposite of pleiotropy)

the character varies in a spectrum and does not have discrete varieties

EX: skin color, height

Question 15

What is the difference between dominantly inherited disorders and recessively inherited disorders?

Answer 15

Dominantly inherited disorders require only one copy of the allele (defective gene) in order for the disorder to be expressed while recessively inherited disorders require two copies

Question 16

Chromosome theory of inheritance

Answer 16

Mendelian genes have specific loci (locations) along chromosomes, and it is the chromosomes that undergo segregation and independent assortment

Question 17

Where are sex-linked genes found?

Answer 17

They are located on a sex chromosome (X or Y in humans)

Question 18

Wildtype

Answer 18

The phenotype for a character most commonly observed in natural populations (EX: red eyes in drosophila)

Alternative traits are called mutant phenotypes

Question 19

Why does X inactivation occur and how does it work?

Answer 19

PURPOSE
Allows males and females to both have one active copy of X-linked genes, so they both produce the same amount of proteins

PROCESS
Females inherit two X chromosomes but one of them is randomly chosen in each cell of the body to become inactivated during embryonic development. This occurs through methylation.

The inactive chromosome condenses into a Barr body, which lies along the inside of the nuclear envelope. (# of barr bodies = total chromosomes for a trait - 1)

However, they are reactivated in the cells that give rise to eggs so that every female gamete will have an active X chromosome after meiosis.

All mitotic descendants of the cell have the same inactive X chromosome

Question 20

Where are linked genes found? What unique pattern of inheritance do they exhibit?

Answer 20

Linked genes are located on the same chromosome and therefore tend to be inherited together in genetic crosses

EX: red hair and freckles

Question 21

Definition of genetic recombination

Answer 21

Production of offspring with a new combination of genes not seen in the P generation (nonparental phenotypes)

This is due to crossing over during prophase I of meiosis

Question 22

What are the two types of offspring phenotypes created by genetic recombination?

Answer 22

Parental phenotypes: offspring with phenotypes the same as one of the parents

Recombinants: offspring with phenotypes different from either parent

Question 23

How does recombination frequency relate to linked genes?

Answer 23

50% or more recombinant frequency = more crossing over = genes are located on different chromosomes and are inherited independently

less than 50% recombinant frequency = less crossing over = genes are located on the same chromosome and are linked

Question 24

Why do linked genes exhibit limited crossing over?

Answer 24

Since linked genes are closer together, there are fewer points for crossing over to occur

Question 25

What is the formula for recombinant frequency?

Answer 25

recombinant frequency = (# of recombinants / total offspring) x 100%

Question 26

How does the distance between genes relate to recombinant frequency?

Answer 26

The farther apart two genes are, the higher the probability that crossing over will occur between them, and thus the higher the recombinant frequency

Question 27

Map unit

Answer 27

Used to express relative distances between genes along a chromosome on a linkage map

1 map unit = 1% recombinant frequency

Question 28

List the four types of chromosomal mutations.

Answer 28

1. Deletion
2. Duplication
3. Inversion
4. Translocation

Question 29

Deletion

Answer 29

Occurs when a chromosomal fragment is lost, resulting in a chromosome with missing genes

Question 30

Duplication

Answer 30

Occurs when a chromosomal segment is repeated, which can alter gene dosage and be harmful

Question 31

Inversion

Answer 31

Occurs when a chromosomal fragment breaks off and reattaches to its original position in reverse orientation

ABC –> CBA

Question 32

Translocation

Answer 32

Occurs when chromosomal fragments of nonhomologous chromosomes swap

Question 33

What is the difference between chromosomal mutations and nucleotide mutations?

Answer 33

Chromosomal mutations are much more large-scale because they involve entire regions of DNA and numerous genes. Nucleotide mutations oftentimes only deal with one base.

Question 34

How do you determine the inheritance pattern of a trait by looking at a pedigree?

Answer 34

1. Sex-linked or Autosomal

Observe if the trait is inherited by both males and females or only one sex. If there is no distinct preference, it’s autosomal

2. Dominant or Recessive

If both parents have a recessive phenotype, all of their offspring should also have a recessive phenotype. If both parents do not have the trait (heterozygous carrier) and the child does, it is recessive.

If the trait is dominant, one of the parents of the affected organism must have had the trait.

Question 35

What is the phenotypic ratio of a dihybrid cross between individuals heterozygous for both traits? (assume complete dominance)

Answer 35

9:3:3:1

Question 36

What is the phenotypic ratio of a monohybrid cross between heterozygotes? (assume complete dominance)

Answer 36

3:1

Question 37

If there is sex-linked recessive inheritance and there is an affected male, which parent donated the affected allele?

Answer 37

Mother

For sons, the father donates the Y chromosome while the mother donates the X chromosome

Question 38

What is the phenotypic ratio of a dihybrid cross between one heterozygote and one homozygous recessive individual? (assume complete dominance)

Answer 38

1: 1:1:1

* if this is not the case, the genes are LINKED

Question 39

Explain how it is possible to have a chromosome that includes BOTH your grandparents’ DNA.

Answer 39

Your mother inherits one member of each homologous pair from your grandparents. During gamete formation in your mother, crossing over occurs between nonsister chromatids in each homologous pair. Thus, each chromosome formed and passed on to you from your mother contains DNA from both your grandparents.