HEREDITY UNIT 5

What is meiosis?

Meiosis is the process that cuts the chromosome number in half to create haploid cells. It ensures formation of haploid gamete cells in s3xua11y reproducing diploid organisms.

Where does meiosis occur vs mitosis in humans?

meiosis occurs in the gonads (testes/ovaries), mitosis occurs in somatic cells (body cells)

Why do chromosome numbers half to be reduced?

because they will be doubled after fertilization

Diploid vs Haploid

Diploid:

• 2 pairs of chromosomes

• pairs differ in size, shape, genetic info, centromere location

• 2n

• body cells are diploid

• cell contain one set from each parent

Haploid:

• 1n

• one set of chromosomes

• gamete cells, s3X cells are haploid

What is an example of meiosis that occurs in females, in males?

Females: oogenesis; creates 1 viable haploid cell, 3 polar bodies (rest are used for nutrients during the travel to implantation) (occurs in follicle cells)

Males: spermatogenesis; creates 4 sperm cells (occurs in seminiferous tubules)

What happens during meiosis I

chromosomes reduced to half

Outline the phases of Meiosis I and II with chromosome number

2n - Interphase - 2n - Meiosis I : Prophase I, Metaphase I, Anaphase I, Telophase I - 2 1n cells - Meiosis II : Prophase II, Metaphase II, Anaphase II, Telophase II - total of 4 1n cells

What occurs during prophase I

• nuclear membrane disappears

• chromatin becomes chromatid

• spindle fibers form

• centrioles form

Two homologues— based on size, shape, genetic info, centromete location— one from each parent come together (process is synapsis) and form a tetrad consisting of 4 chromatids

Chromatids exhibit crossing over resulting in recombinant chromatids

What is crossing over?

When tetrads come together during synapsis, chromosomes may intertwine and break, exchanging segments. This increases genetic diversity as the process is completely random.

What occurs during metaphase I

Spindle fibers move the chromosomes, aligning tetrads at equator. Chromosomes are ‘stacked’ (variable)

What is independent assortment?

Assortment of chromosomes are random, the ordering of the homologous pairs (tetrads) affect which chromosome ends up in each gamete.

What occurs during anaphase I

Fibers separate chromosome pairs, each pair of sister chromatid migrate to opposite sides of the cell

What occurs during telophase I

• nuclear membrane forms (2 separate nuclei)

• each nucleus contains only one double chromosome from each pair, haploid

• chromosomes become chromatin

• cytokinesis occurs, 2 daughter cells each haploid

What occurs during prophase II

• nuclear envelope disappears, spindle fibers form

(no interphase inb/w telophase I and prophase II)

What occurs during metaphase II

• Chromosomes are pulled to equator by spindle fibers

• no stacking (no tetrads)

What occurs during anaphase II

• chromosomes pulled to opposite sides of cell

• centromeres are broken, each sister chromatid separates into individual chromosomes

What occurs during telophase II

• nuclear envelope reappears

• each nucleus contains single chromosomes

• chromosomes into chromatids

• cytokinesis separates the two cells into four daughter cells, each haploid

Compare mitosis vs meiosis

Similarities:

• nuclear envelope disappearing

• DNA coiling to chromosomes (Chromatids)

• Aligning chromosomes at equator

• Using spindle fibers to separate chromosomes

• Nuclear envelope reappearing

• Chromosomes uncoiling (chromatin)

• Cytokinesis and the production of daughter cells

Differences:

• Mitosis produces 2 daughter cells genetically identical to the parent. If cell starts as diploid it ends as diploid, if haploid end in haploid. (occurs in somatic cells)

• Meiosis produces 4 haploid daughter cells, genetically varied from each other and the diploid parent cell (occurs in gonads)

• Mitosis has 1 round of division, meiosis has 2

What factors contribute to variation?

• Independent assortment (stacking)

• Crossing over (synapsis, exchanging of genetic info between homologues)

• Random fertilization (which sperm fertilizes the egg)

What are the carriers of genetic information?

DNA and RNA

What is a factor of common ancestry?

presence of ribosomes

What is the process all organisms use?

cellular respiration → glycolysis

What is a gene?

a heredity coding for a trait that can be transferred from one generation to another. Made up of a specific strand of DNA, made up of chromosomes

What is a trait? What determines it?

a trait is a genetically determined characteristic of an organism.

What is the loci?

the location of the gene on the chromosome

What are alleles? Give examples of the two types of alleles and describe them

Alleles are different variations of genes that organisms inherit from their parents. There is the dominant allele that always shows up in the phenotype if inherited. The recessive allele only shows in the phenotype if the dominant allele has not been inherited.

What is a genotype? Compare with what a phenotype is

A genotype is the combination of the inherited alleles (actual gene) . If they are the same it is homozygous/purebred, if they are different it is heterozygous/hybrid. A phenotype is what is physically expressed due to the genotype. ex: R- widows peak r- no widows peak genotype = Rr ..phenotype= widows peak

What are the three generations? (brief explanations)

P Generation - parental generation

F1 - offspring of parental generation

F2- offspring of 2 F1’s

Describe Mendel’s 4 (5) laws of heredity

1. Law of unit characters: inheritance of each trait is determined by units (genes) passed from descendants

2. Law of dominance: the dominant allele will always mask the recessive, one allele does not alter the other and either one can be passed to the next gen. unchanged

3. Law of segregation: For any trait, the pair of alleles of each parent separate and only one allele is passed from each parent to an offspring (matter of chance); occurs during meiosis

4. Law of independent assortment: Traits can be inherited independently of other traits; if u have brown eyes u dont automatically have black hair

5. law of probability: applied to analyze passage of single gene traits from parent to offspring

• to determine probability of A or B happening, add together

• to determine probability of A and B happening, multiply together

What is a monohybrid cross?

examination of how one trait is inherited

What are the examples of non-mendelian laws?

• Incomplete dominance

• codominance

• multiple alleles

• polygenic traits

• linkage

• s3x linked traits

• non-nuclear inheritance

What is incomplete dominance?

the offspring shows a mixture of two traits, neither allele is dominant.

What is codominance?

both traits are expressed, like a combination of alleles

What is the main difference between codominance and incomplete dominance?

codominance: combined alleles together (heterogenous mixture) ex: red bull + white bull = roan bull

incomplete dominance: blending alleles (homogeneous solution) ex: Red flower x White flower = pink flower

What are multiple alleles?

When there are more than two alleles for a particular gene within a POPULATION. An individual can only possess two alleles.

What is a dihybrid cross?

an examination of how two traits are inherited.

What is a pedigree? What does it help identify?

A visual representation tracing the history of a trait through familial generations. Helps identify types (modes) of inheritance

What are Autosomal Dominant traits

Only one copy of the dominant allele is needed for a person to exhibit the trait. (no carriers)

• May be homozygous or heterozygous

• Males and females can be affected (MLM transmission is possible lel)

• Males and females transmit the trait at equal frequency

• No generations skipped

• Transmission can stop when no family members are affected

What are autosomal recessive traits?

two copies of the recessive allele are needed for a person to exhibit the trait

• IF one copy is present, the person is a carrier

• Males and females affected

• can transmit the trait ^

• trait can ‘skip’ generations (only carriers present within the generation)

What are X-linked recessive traits?

• Males always express the trait

• Females have to be homozygous to express trait

• Affected females must have an affected father and a mother who is affected or heterozygous

• trait can skip generations

What are X-linked dominant traits?

• Only one copy of the allele is needed to have the trait

• the allele is found on X chromosome

• Affected fathers cannot pass the allele to their sons cuz they only pass the Y chromosome to sons

What are Y-linked traits?

• allele is for trait found only on Y chromosomes

• Fathers who possess the trait will always pass it to their son

• females never get the trait