chapter 4 - Genetics

The DNA in eukaryotes is associated with proteins which help keep the DNA organized.

• gene = a heritable factor that controls a specific characteristic

• allele = one specific form of a gene, differing from other alleles by one or a few bases only and occupying the same gene locus as other alleles of the gene

• genome = the whole of the genetic information of an organism

a gene mutation is a change in the base sequence of a gene

1. Base substitution --> from A to T in the triplet coding for the sixth amino acid. The mutation changes HbA into new allele HbS.

2. One codon in the mRNA is different and therefore one amino acid in the polypeptide is altered.

3. In some conditions red blood cells containing the altered hemoglobin become sickle shaped.

Living organisms that reproduce sexually have to halve their chromosome number at some stage in the life cycle because the fusion of gametes during fertilization doubles it.

chromosomes with the same gene loci in the same sequence which are capable of pairing up to form bivalents during the first prophase of meiosis.

Homologous chromosomes have the same genes as each other, in the same sequence, but not necessarily the same alleles of those genes.

1 diplod parent cell --> 4 haploid daughter cells. During interphase DNA replication --> each chromatid have an identical copy (sister chromatid) attached at centromere. Crossing over different from Mitosis; allows DNA from mother to mix with DNA from father.

Prophase 1

• chromosomes have become visible
• homologous chromosomes pair up
• crossing over occurs
• spindle fibres made from microtubules form

Metaphase 1

• the pairs of chromosomes line up on the equator
• the nuclear membrane disintegrates

Anaphase 1

• spindle fibres from the poles attach to chromosomes and pull them to opposite poles of the coil.

Telophase 1

• spindle fibres disintegrates
• chromosomes uncoil and new nuclear membranes form
• (many plants do not have this stage)

Cytokinesis occurs --> 2 cells. Haploid because only one chromosome of each pair. However, each chromatid still has its sister chromatid attached.

Prophase 2

• DNA visible
• spindle fibre produced

Metaphase 2

• nuclear membranes disintegrates
• individual chromosomes line up along equator of each cell in no special order. RANDOM ORIENTATION
• spindle fibres attach to each of the sister chromatids at the centromere

Anaphase 2

• centromeres of each chromosome split --> sister chromatids --> individual chromosome
• spindle fibre pull individual chromosomes

Telophase 2

• Chromosomes unwind their strands of DNA
• Nuclear envelopes form around each of the 4 haploid cells, preparing them for cytokinesis.

non-disjunction: a process by which two or more homoloug chromosomes stick together instead of separating in Meiosis.

Down's Syndrome: 21st pair --> 3 instead of 2. Called trisomy. --> malfunction of digestive system and learning difficulties.

Affects 1 in 800 births, increase with age.

in karyotyping, chromosomes are arranged in pairs according to their size and structure

Amniocentesis: a sample of amniotic fluid is removed from the amniotic sac around the fetus using a hypodermic needle. Contains cells from the fetus.

Chorionic villus sampling: Cells are removed from fetal tissues in the placenta called chorionic villi, by a  needle.

gender: 23d pair

non-disjunction: eg, 3 chromosomes in a pair

Genotype: The symbolic representation of alleles possessed by an organism.

Phenotype: the characteristics or traits of an organism

Dominant allele: An allele that has the same effect on the phenotype wheter it is paired with the same allele or a different one.

Recessive allele: An allele that has an effect on the phenotype only when present in homozygous state.

codominant alleles: Pairs of alleles that both affect the phenotype only when present in a heterozygote.

Locus: the particular position on homologous chromosomes of a gene.

Homozygous: Having two identical alleles of a gene.

Heterozygous: Having two different alleles of a gene.

Carrier: An individual who has a recessive allele of a gene that does not have an effect on their phenotype.

Test cross: Testing a suspected heterozygote by crossing it with a known homozygote recessive.