GeneticsGenetics4.2 & 10.1 Meiosis

MeiosisMeiosis

Diploid – two sets of each chromosome present◦In humans, where can you find diploid cells?

Haploid – one set of each chromosome present◦ In humans, where can you find haploid cells?

Meiosis is a reduction division. This means that a diploid nucleus is divided into 4 haploid nuclei.

These become the nuclei for gametes (egg and sperm cells).

Homologous chromosomeHomologous chromosome

Homologous chromosomes are the chromosomes that are the same size and show the same banding pattern. They contain the same set of genes but their alleles are not identical.

MeiosisMeiosis

Summary of Meiosis:◦Meiosis occurs after interphase ◦ (replication has occurred).◦There are 2 divisions of the nucleus (each with prophase, metaphase, anaphase, telophase, cytokinesis).

◦The homologous chromosomes are paired in prophase 1.

◦The result of the process is four haploid cells.

Crossing OverCrossing Over

Crossing over (i.e. recombination or synapsis) allows for genetic variability

Only happens in meiosis (prophase I)Forms a chiasmata between non-sister

chromatids on homologous chromosomes.

1. Homologous chromosomes pair loosely along their lengths

2. Homologues align with each other gene by gene.3. Homologous portions of 2 non-sister chromatids

trade places (2-3 per chromosome pair for humans)

4. Locations where these genetic exchanges have occurred are visible as chiasmata.

Crossing overCrossing over

Genetic VariabilityGenetic Variability

Gametes have genetic variability through◦ Crossing over during prophase I◦ Independent Assortment- Random orientation during

metaphase IMore genetic variability comes with random

fertilization of gametes.

Egg x Sperm(223 combinations of chromosomes) x (223 combinations of chromosomes)

223 x 223 diploid combinations

1 in 70 trillion chance for the same genes- not even considering crossing over.

Non-DisjunctionNon-Disjunction

Non-disjunction occurs when the centromeres do not uncouple which leads to one nucleus with a missing chromosome and one nucleus with an extra chromosome.

This is what causes genetic disorders like Down Syndrome (trisomy 21)

KaryotypingKaryotyping

A karyotype is an image of a set of chromosomes that is taken during metaphase. The chromosomes are lined up in homologous pairs based on size and structure.

KaryotypingKaryotyping

Karyotyping is performed using cells located by chorionic villus sampling or amniocentesis. This allows pre-natal (before birth) diagnosis of chromosome abnormalities.

Genetic Screening Using Genetic Screening Using KaryotypingKaryotyping

Karyotyping can be used to screen people for genetic diseases.

This allows for:◦The probability of a couple having a child with

genetic disease can be determined beforehand.◦Fewer children to be born with genetic diseases

because fetuses can be aborted or people carrying the diseases can avoid reproduction.

Therefore there can be;◦Less cost for long-term health care◦Less frequency of harmful alleles in the gene

pool

Genetic Screening Using Genetic Screening Using KaryotypingKaryotyping

Early detection of a genetic disease in a fetus also allows…

◦Treatment to begin early for the genetic disease

◦Parents of affected children can be prepared psychologically for the arrival of their baby.

Questions about KaryotypingQuestions about Karyotyping

Risks of side effects from karyotyping (e.g. abortion of the fetus).

Who should make the decisions about karyotyping and abortion?

Should national governments interfere with personal freedom to abort fetuses that have genetic disease?

Should abortion based on gender be allowed? How could it be stopped?

Analyze this karyotype:Analyze this karyotype:

What is the gender? Has non-disjunction occurred?

Analyze this karyotype:Analyze this karyotype: