BIOL 1201 Exam 3 (The Chromosomal Basis of Inheritance)

▪ A larger X chromosome

▪ A smaller Y chromosome

What are the two types of sex chromosomes humans and other mammals have?

usually develop as female

Individuals who inherit two X chromosomes (XX)

usually develop as male.

Individuals who inherit one X and one Y chromosome (XY)

Homologous

- Refers to structures or chromosomes that share a common ancestry or genetic similarity, despite potentially serving different functions or being present in different species

Homologous/ Homologous

Short segments at either end of the Y chromosome are ________ to corresponding regions on the X chromosome.

- These regions allow the X and Y chromosomes in males to:

▪ Pair during meiosis in the testes

▪ Behave like ________ chromosomes

X-0 System

Sex determination among animals:

o Found in some insects. o Females: XX

o Males: X (only one X chromosome)

Z-W System

Sex determination among animals:

o Found in birds, some fishes, and some insects. o Females: ZW

o Males: ZZ

Haplo-Diploid System

Sex determination among animals:

o Found in bees and ants. o Females: Diploid (develop from fertilized eggs)

o Males: Haploid (develop from unfertilized eggs)

Haploid cells

- develop from unfertilized eggs

- A cell containing only one set of chromosomes (n).

Diploid

- develop from fertilized eggs

- A cell containing two sets of chromosomes (2n), one set inherited from each parent.

This gene was named SRY (sex-determining region of Y)

- A gene that was identified on the Y-chromosome by a British research team in 1990

- Essential for the development of testes

Testes

The male reproductive organ, or gonad, in which sperm and reproductive hormones are produced.

- In individuals with the SRY gene, the embryonic gonads develop into testes.

- In individuals without the SRY gene, the embryonic gonads develop into ovaries.

What is the function of the SRY gene?

Sex-linked gene

A gene located on either sex chromosome

smaller

In the X-Y system, the Y chromosome is much ______ than the X chromosome.

Y-linked genes

Genes on the Y chromosome are known as?

78 genes; 25 proteins

The human Y chromosome contains about ___ genes that code for approximately ___ proteins.

X-linked genes

Genes on the X chromosome are called?

1,100 genes

The human X chromosome contains about ______ genes.

X chromosome

- Carries genes for many traits unrelated to sex.

Y-linked genes

- genes specifically related to sex determination.

• A female must have two copies of the recessive allele (homozygous). (XX)

• A male needs only one copy of the recessive allele (hemizygous), since he has only one X chromosome. (XY)

For a recessive X-linked trait to be expressed:

because males have only one X chromosome.

Why are X-linked recessive disorders much more common in males?

Heterozygous females

- (with one normal and one recessive allele) are considered carriers of the trait (X-linked recessive disorder)

- They typically do not show symptoms, but they can pass the trait to their children

• Color blindness (mostly X-linked)

• Duchenne muscular dystrophy

• Hemophilia

What are some disorders caused by recessive alleles on the X chromosome in humans?

Duchenne muscular dystrophy

▪ The disease is characterized by a progressive weakening of the muscles and a loss of coordination.

▪ This disorder is due to the absence of an X-linked gene for a key muscle protein called dystrophin.

Hemophilia

▪An X-linked recessive disorder defined by the absence of one or more proteins required for blood clotting.

▪These proteins normally slow and then stop bleeding. ▪Individuals with hemophilia have prolonged bleeding because a firm clot forms slowly.

▪Bleeding in muscles and joints can be painful and can lead to serious damage.

X-chromosome Inactivation in Females

• In mammalian females, one of the two X chromosomes in each cell is randomly inactivated during embryonic development. • This process ensures that females have only one functional X chromosome per cell (dosage compensation).

inactivated

• In mammalian females, one of the two X chromosomes in each cell is randomly ________ during embryonic development.

Barr body

- The inactive X chromosome condenses into a dense, compact structure called a...?

- Remains inactive throughout the cell's lifetime and is reactivated only in cells that form gametes (eggs).

XIST (X-inactive specific transcript) gene

- A gene that becomes active only on the chromosome that will be inactivated.

- produces an RNA molecule that coats the X chromosome, leading to its condensation and inactivation.

- ensures that only one X chromosome remains active in each somatic cell.

Mosaic

If a female is heterozygous for a gene located on the X chromosome, she becomes a _____ for that trait.

- means some cells express the allele from one X chromosome, while others express the allele from the other X chromosome.

Linked genes

Genes that are located on the same chromosome tend to be inherited together and are called..?

Parental types

- A type of offspring based on Phenotype

- Offspring whose phenotype matches one of the parental (P) generation phenotypes.

Recombinant types (recombinants)

- A type of offspring based on Phenotype

- Offspring with new combinations of traits that do not match either parental phenotype.

Frequency of Recombination

(50%) and (meiosis)

• A ___% frequency of recombination is observed for any two genes located on different (nonhomologous) chromosomes.

• This occurs because the genes assort independently during ______.

Testcross

- Produced four phenotypes among the offspring

- Breeding an organism of unknown genotype with a homozygous recessive individual to determine the unknown genotype.

- The ratio of phenotypes in the offspring reveals the unknown genotype.

Parental types (50%)

▪ Yellow-round seeds (YR)

▪ Green-wrinkled seeds (yr)

Recombinant types (50%)

▪ Yellow-wrinkled seeds (Yr)

▪ Green-round seeds (yR)

What would be the results of the testcross of F1 plant (YyRr) wth a homozygous recessive plant (yyrr)?

leads to the independent assortment of alleles.

- The random orientation of homologous chromosomes during metaphase I of meiosis leads to the _____ _____ ___ ____.

• This random alignment produces new combinations of alleles in gametes.

Nondisjunction

An error in meiosis or mitosis in which members of a pair of homologous chromosomes or a pair of sister chromatids fail to separate properly from each other.

- As a result, one gamete receives two of the same type of chromosome, and another gamete receives no copy

Aneuploidy

- a chromosomal abnormality which results from the fertilization of gametes in which nondisjunction occurred

- Offspring with this condition have an abnormal number of a particular chromosome.

Monosomic Zygote

- A zygote that has only one copy of a particular chromosome and have 2n − 1 chromosomes (missing one chromosome).

Zygote

The diploid cell produced by the union of haploid gametes during fertilization; a fertilized egg.

Trisomic

A zygote has three copies of a particular chromosome and have 2n + 1 chromosomes (having one extra chromosome)

Recombinant chromosomes

- Chromosomes that bring alleles together in new combinations in gametes

Random fertilization

- A type of fertilization that increases even further the number of variant combinations that can be produced

Polyploidy

- A condition in which an organism has more than two complete sets of chromosomes

Triploidy (3n)

three sets of chromosomes

Tetraploidy (4n)

four sets of chromosomes

Deletion, Duplication, Inversion, Translocation

Breakage of a chromosome can lead to four types of changes in chromosome structure. What are these four changes?

Deletion

- A change in chromosome structure that removes a chromosomal fragment

Duplication

- A change in chromosome structure that repeats a segment

Inversion

- A change in chromosome structure that reverses orientation of a segment within a chromosome

Translocation

- A change in chromosome structure that moves a segment from one chromosome to another

Meiosis

Deletions and duplications are especially likely to occur during ______.

essential genes

- this is usually lethal

A diploid embryo that is homozygous for a large deletion or a male with a large deletion to its single X chromosome is usually missing many _____ _____.

- Human disorders are due to Chromosomal alterations

- Alterations of chromosome number and structure are associated with some serious disorders

- Some types of aneuploidy appear to upset the genetic balance less than others, resulting in individuals surviving to birth and beyond

What causes human disorders?

Down Syndrome

- an aneuploid condition that results from three copies of chromosome 21

- The frequency increases with the age of the mother, a correlation that has not been explained

Klinefelter syndrome

- A syndrome that is the result of an extra chromosome in a male, producing XXY individuals

- XXX females occur with a frequency of about one in 1,000

They are healthy but are at risk for learning disabilities

Monosomy X, called Turner syndrome

- Produces X0 females, who are sterile; it is the only known viable monosomy in humans

Cri Du Chat ("cry of the cat")

- A syndrome that results from a specific deletion in chromosome 5.

- A child born with this syndrome is severely intellectually disabled and has a catlike cry; individuals usually die in infancy or early childhood

chronic myelogenous leukemia (CML)

- A cancer where too many blood cells made in bone marrow

- occurs when a large fragment of chromosome 22 switches places with a small fragment from the tip of chromosome 9.

- caused by translocations of chromosomes

Philadelphia chromosome.

The resulting short and easily recognized chromosome 22 is called the ...?