Genetics

Biology Chapter 10 Notes

Chapter 10 Voc. Word list (includes People):

People: John Dalton and Gregor Mendel

Voc. Words: Meiosis, heredity, traits, gametes, fertilization, zygote, pollination, alleles, Punnett Square, Dominant factor, Recessive factor, Law of segregation, Law of Independent Assortment, phenotype, genotype, homozygous, heterozygous, hybrid, gametogenesis, reductional division, equational division, synapsis, bivalent, tetrad, chiasma, crossing over, Interkinesis, nondisjunction, trisomy, monosomy, and polyploidy

Review:

  • Part of the cell theory by John Dalton states that all cells come from preexisting cells.

Background:

New cells form by binary fission, mitosis, or meiosis.

  • No matter what the process, the new cells receive chromosomes from the original cell.
  • They may receive either a full set or half of what was present.

When Meiosis occurs the chromosomes will be transmitted from the parent to the offspring.

  • Chromosomes contain the information that determines an individual’s characteristics
  • Meiosis is covered under the field of Biology called Geneticswhich deals with heredity-the passing on of the characteristics from parents to offspring.
  • The Father of Genetics was an Austrian monk named Gregor Mendel

Mendelstudied several characteristics of garden peas in the mid 1800’s.

  • Characteristics that are inherited are called traits.
  • Parental generation –P generation
  • Offspring of the P generation make up F1 (first filial) generation-In humans that would be children
  • Offspring of the F1 generation make up the F2 generation-In humans that would be grandchildren

Mendel discovered that that garden peas reproduce sexually, which means that they produce male and female sex cells, called gametes.

In this organism the male gamete is in the form of pollen grain, which is produced in the male reproductive organ.

The female gamete forms in the female reproductive organ.

In the process calledfertilization, the male gamete unites with the female gamete. The resulting fertilized cell is called a zygote, that later develops into a seed.

The transfer of pollen grains from the male reproductive organ to a female reproductive organ is called pollination.

Mendel‘s experiments led to several conclusions.

  1. Each trait is controlled by two factors-also called alleles(alternative from of a gene represented as letters in Punnett Square)
  2. Punnett Square is often used to show how traits are inherited.

A.)Dominant factor -Capital letter/s

  • Example: YY or Yy

B.)Recessive factor-small cap letters

  • Example: yy
  1. Factors segregate, or separate, when a gamete forms-Law of segregation
  2. Factors for different characteristics are independently distributed to a gamete-Law of Independent assortment

Punnett Square is used to determine the offspring of two parents.

  • The phenotype is the appearance of a trait in an organism
  • Example Black coat color of a rabbit
  • Genotype is the letters that represent the trait.
  • Example Black coat color of a rabbit (phenotype) and genotype is BB
  • You have homozygousgenotypes and heterozygous types
  • In the case which the Rabbit coat color there are three possible genotypes possible: BB Bb, and bb
  • Both BB (Black) and bb (Brown) are homozygous, where both pairs are alike.
  • Bb (Black) is heterozygous, where both alleles are different.
  • The Punnett Square crossing of these two parents is one trait-coat color, and is called a Monohybrid Cross
  • A hybrid is the offspring of parents that have different forms of a trait.

Bb

BB / Bb
Bb / bb

B

b

The alleles contributed by one parent are placed along the top. The alleles contributed by the other parent are placed along the side.

  • Each box represents a combination that is a possible genotype in the offspring.
  • In this cross, the probabilities are 25% homozygous black individuals (BB), 50 % heterozygous black individuals (Bb), and 25% homozygous brown individuals (bb).

A cross that involves two traits is called a Dihybrid cross.

In this case Round (R) is dominant to wrinkled (r). Yellow (Y) is dominant to green (y). The genotypes for both parents are RrYy. The following Punnett Square illustrates the possible combinations of genotypes in F1generation.

RYRyrYry

RRYY / RRYy / RrYY / RrYy
RRYy / RRyy / RrYy / Rryy
RrYY / RrYy / rrYY / rrYy
RrYy / Rryy / rrYy / rryy

RY

Ry

rY

ry

Notice that four phenotypes are possible among the offspring: (1) round, yellow seeds; (2) round, green seeds; (3) wrinkled, yellow seeds, and (4) wrinkled, green pea seeds.

What are the nine genotypes and percentages?

What are the four phenotypes represented?

Answer:

RRYY 1/16 6.25%Round , Yellow seed

RRYy 2/16 12.50%Round ,Yellow seed

RRyy1/16 6.25%Round ,green seed

RrYY2/16 12.50%Round ,Yellow seed

RrYy4/16 25%Round, Yellow seed

Rryy2/16 12.50%Round , green seed

rrYY1/16 6.25%Wrinkled ,Yellow seed

rrYy2/16 12.5%Wrinkled ,Yellow seed

rryy1/16 6.25%Wrinkled, green seed

10.2 Meiosis

Meiosis

Sexual reproduction involves the manufacture of gametes –gametogenesis

The union of a male and female gamete is called –fertilization

  • Fertilization produces a zygote
  • Male gamete- sperm
  • Female gamete-egg or ovum
  • The zygote then develops by mitosis into a multicellular organism
  • The pattern of reproduction, involving the production and subsequent fusion of haploid sex cells, is called sexual reproduction

Gametogenesis

  • Occurs only in specialized cells (germ line) of reproductive organs (gonads)
  • Male-testes
  • Female-ovaries
  • Gametes contain the haploid # (n) of chromosomes, but originate from diploid (2n) cells of the germ line.
  • The 2 chromosomes of each pair in a diploid cell are called homologous chromosomes
  • Each of a pair of homologous chromosomes has genes for the same traits, such as plant height.
  • The genes are arranged in the same order, but because there are different alleles for the same gene, the homologous pair are not always identical to each other
  • The number of chromosomes must be reduced by half to maintain the chromosome number characteristic of the species through meiosis.

Meiosisinvolves a single DNA replication and 2 divisions of cytoplasm. (Both consist of 4 major phases: prophase, metaphase, anaphase, and telophase)

  • 1st Meiotic division (Meiosis I) is a reductional division that produces 2 haploid cells from a single diploid cell.
  • 2nd Meiotic division (Meiosis II) is an equational division. In this case, sister chromatids of haploid cells are separated.

Meiosis I

  • DNA replicates during Interphase
  • Prophase I
  • Each pair of homologous chromosomes comes to lie side by side in a pairing process called synapsis. Each pair of synapsed chromosomes is called a bivalent (2 chromosomes)
  • Each chromosome consists of 2 identical sister chromatids called tetrad (4 chromatids)
  • During synapsis non-sister chromatids (one from each paired chromosomes) of tetrad may break and reunite at one or more corresponding sites in a process called crossing over. The point of exchange is called chiasma (chiasmata, plural) where either 2 or 4 chromatids cross over.
  • Divided in 5 stages
  • Leptonema (thin-thread stage) –long, thin chromosomes start to condense and threadlike structures appear from the nuclear chromatin material
  • Zygonema (joined-thread stage)-synapsis begins
  • Pachynema (thick-thread stage), synapsis appears so tight that it becomes difficult to see in a bivalent
  • Diplonema (double-thread stage) –see individual chromatids and chiasmata forms
  • Diakinesis –the chromosomes reach maximum condensation, nucleoli and nuclear membrane disappear, and spindle apparatus begins to form.
  • Metaphase I-bivalents are on the equatorial plane
  • Anaphase I-centromeres do not divide, but continue to hold sister chromatids together.
  • If cross-overs occur sister chromatids may not be identical-genetic recombination
  • It is the major source of variation among organism
  • Variation is important to a species because it is the raw material that forms the basis for evolution
  • Move to opposite poles
  • Chromosomes divide to haploid (n) state
  • Telophase I-Cytokinesis occurs(divides mother cell into 2 haploid daughter cells

End of Meiosis I

Interkinesis –chromosomes uncoil and return to Interphase like condition, but chromosomes again condense and nuclear membrane will disappear

  • No DNA replication

Meiosis II

  • Prophase II-spindle apparatus reforms
  • Metaphase II-individual chromosomes line up at the equatorial plane
  • Anaphase II-centromeres of each chromosome divides allowing the sister chromatids to be pulled apart in an equational division (mitosis-like) by spindle fibers.
  • Telophase II-Cytokinesis divides each cell into 2 progeny cells
  • So a diploid mother cell becomes 4 haploid progeny cells

Problems can occur because homologous chromosomes fail to separate properly during meiosis-nondisjunction

  • During Meiosis I, one chromosome from each homologous pair moves to each pole of the cell. In this case, both chromosomes of a homologous pair move to the same pole of the cell.
  • In one type of nondisjunction, two kinds of gametes result cause an extra chromosome to form on one, and missing a chromosome on the other.
  • Condition: Trisomy
  • The resulting zygote has 47 chromosomes instead of 46 –Down syndrome
  • Condition: Monosomy
  • If the case of missing one most zygotes die.
  • But if the zygote survives and develops it can have Turner syndrome
  • In humans, females only have one X chromosome instead of two
  • Condition: Polyploidy-With more than usual sets of chromosomes
  • Rare in animals and almost always causes death of the zygote

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