CH 16 INHERITANCE

• Division of the nucleus takes place in two ways: mitosis and meiosis

Mitosis

• Produces two daughter nuclei with the same number of chromosomes as the parent.

Meiosis

• Produces four daughter nuclei, each with half the number of chromosomes of the parent.
• Maintains the number of chromosomes in adults of a species constant.
• Each adults cell has a fixed number of chromosomes, usually in pairs that carry the same genes.
• One chromosome of each pair is maternal, the other is paternal.
• Pairs of chromosomes are known as homologous pairs, with the total number referred to as the diploid number.
• During meiosis, each member of a homologous pair is separated, so only one enters each daughter cell.
• This is known as the haploid number.
• When two haploid gametes fuse, the diploid number is restored.
• All diploid cells have two sets of chromosomes, one from each parent giving two sets of information.
• Any two chromosomes that have the same genes are termed a homologous pair.
• Genes may code for round, yellow seeds in plants while other genes code for wrinkled, green seeds.
• The halving of chromosome number during meiosis isn't random but ensures each daughter cell receives one of each homologous pair.
• When haploid cells combine, the diploid state, with paired chromosomes, is restored.

Gametogenesis

• Gametogenesis is the formation of haploid reproductive cells.
• In humans, gametes originate from diploid germ cells multiplying by mitosis to build up parent cells.
• These divide by meiosis to give mature haploid gamete, which, when combined with a gamete from the opposite sex restores diploid state.
• In males, the process is spermatogenesis; in females, it is oogenesis.
• In plants, the process is more complex.
• The diploid mother cell divides by meiosis to give haploid cells further dividing by mitosis to increase numbers.
• Nuclear division isn't always followed by cell division.
• The female plant part produces embryo sacs w/ eight haploid nuclei, while the male part produces pollen grains each with two haploid nuclei.
• In late interphase before meiosis, DNA replicates, doubling the chromosome copies.
• In animal cells, the centriole pair replicates during prophase I.

Prophase I

• Chromosomes shorten and come together in homologous pairs to form a bivalent, condensing.
• Chromatids wrap around one another, attaching at chiasmata, and non-sister chromatids swap sections in crossing over.
• Nucleolus disappears and the nuclear envelope disassembles.

Metaphase I

• Centromeres attach to the spindle and bivalents arrange randomly on the cell equator.
• Each member of a pair of homologous chromosomes faces opposite poles.

Anaphase I

• One of each pair of homologous chromosomes is pulled to opposite poles by spindle fibres.

Telophase I and Cytokinesis

• Microtubules pull two sides of the cell surface membrane together, constricting the cell until it divides into two.
• Nuclear envelope reforms around chromosomes in animal cells; in plants, the cell transitions directly into metaphase II.
• Meiosis involves two nuclear divisions.
• The first meiotic division (meiosis I) has four stages: prophase I, metaphase I, anaphase I, and telophase I.
• In prophase I, homologous pairs form a bivalent in synapsis and chromatids wrap around forming chiasmata where crossing over occurs.
• The second meiotic division (meiosis II) is a mitotic division: sister chromatids separate and move to opposite poles.
• The four stages occur simultaneously in the daughter cells from meiosis I, resulting in four daughter cells.

Meiosis II

• Reduces chromosome number
• Crossing over during prophase I creates genetic variation.
• During prophase II, the nuclear envelope breaks if it already formed, nucleolus disappears, chromosomes shorten and thicken, and the spindle re-forms.
• Chromosomes arrange during metaphase II at the equator, chromatids move towards poles during anaphase II, and telophase II produces four daughter cells.
• Daughter cells have ½ the number of chromosomes, genetically unique due to crossing over.
• Meiosis halves chromosome number and maintains diploid number when gametes fuse, produces genetic variation allowing organisms to adapt and survive.
• Genetic variation in three main ways: genetic recombination by crossing over, random assortment of homologous chromosomes, and random fusion of gametes.
• In meiosis 1, each chromosome lines up alongside its homologous partner.
• As chromatids twist around one another tensions are created and portions of the chromatids break off and rejoin.
• Points of crossover are called chiasmata.
• Usually equivalent portions of homologous chromosomes are exchanged.
• New genetic combinations of maternal and paternal alleles are produced.
• Where no recombination by crossing over, only two types of cell produced, but where recombination occurs, four produced.

Metaphase I

• Homologous chromosomes arrange on equator randomly (random assortment).
• Though each pair has the gene some vary in alleles possessed.
• Random assortment produces new combinations.
• Haploid gametes produced fuse randomly restoring diploid state and due to random fusion produces variety.

Genotype

• The genetic make-up of an organism which describes all the alleles that an organism contains.
• Indicates limits characteristics of individual varies.
• Change to the genotype due DNA change called mutation inherited if in gametes.

Phenotype

• Observable characteristics resulting from genotype/environment interaction; change not affecting phenotype is modification not inherited.
• Indicates particular gene describing combination of alleles.
• Includes observable effects specific gene.
• A gene is a length of DNA (sequence of nucleotide bases) determining something.
• Genes exist in two, or more, forms- alleles.
• The position of a gene on a chromosome = locus.
• Allele = different form of gene, pea plant, gene, seed pod = forms allele green pod, one = yellow.
• 1 allele of gene occur, locus, chromosome and sexually reproducing orgs have homologous pairs of chromosomes.
• loci can carry 1 allele gene.
• Identical alleles= homozygous; w/ different = heterozygous.
• Heterozygous states- only one allele phenotype shows.
• Exhibits in phenotype = dominant.
• Has effect only w/ another identical = recessive.
• Homozygous org w/ 2 dominant= homozygous dominant.
• Org w/ two recessive= homozygous recessive.
• Alleles contribute to phenotype= codominant.
• Only results combine both features.
• Summary terms : The genetic composition of an organism is the genotype.
• A change is called mutation.
• The phenotype appearance is a modification not inherited.
• Gene =sequence, the allele is when it effects.
• The genetic composition of an organism is called the genotype.
• Change = mutation and can inherit, appearance= phenotype & change =modification
• a gene = sequence codes + the 2 or = alleles.
• The pairs of chromosomes are said to same but are different. The recessive can affect.
• Can affect = Dominant allele to the genotype of that org
• Monohybrid inheritance: inheritance of single gene.
• A trait, i.e blood group type, feature measured.
• if pea plants with green repeatedly bred = always produce green = pure breeding
• can produce= pure breeding strains for ANY characteristic homozygous (two same alleles).
• green crossed with pure yellow, offspring= F 1 first filial + all turn OUT
• The allele green= dominant, to yellow = recessive
• G = allele green
• g = allele the
• Pure breeding= dominant x recessive

Recessive

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