GENETIC VARIATION AND INHERITANCE

When inheritance doesn’t go as planned…

Genetic Variation

• Any change in the makeup of the genes of a give species is genetic variation.

• Without random events that alter our genes, new (and sometimes beneficial) traits would never develop.

• As well, no variation means no evolution.

Mechanisms of Variation

• “Crossing Over”• Occurs during meiosis, when

homologous pairs of chromosomes are in contact.

• Segments from one chromosome in the pair can “swap” with segments from the other chromosome.

• Creates new and unpredictable combinations of genes – much more variation in offspring!

http://www.youtube.com/watch?v=KW685fz5Txs

Errors in Meiosis

• If chromosomes separate improperly, new gametes are still produced.

• HOWEVER, this often leads to harmful consequences in offpsring.

• The most common error in meiosis is called nondisjunction.– It occurs when chromosomes do NOT

separate during meiosis and move together into a single gamete.

http://kisdwebs.katyisd.org/campuses/MRHS/teacherweb/hallk/Teacher%20Documents/AP%20Biology%20Materials/Genetics/Meiosis%20Mistakes/12_A02s.swf

Nondisjunction

• Chromosomes can improperly separate during the first stage of meiosis (“meiosis I”) or the second stage (“meiosis II”).

• If it happens during meiosis I, all gametes will be affected (either missing chromosomes or having too many).

• If it happens during meiosis II, some gametes will be affected, and others will end up being normal.

Nondisjunction cont’d

Results of nondisjunction:• Abnormal gametes lead to offspring

with atypical amounts of chromosomes – this is aneuploidy.– Includes monosomy (missing one of the

chromosomes from a pair; you’d have one copy of a chromosome rather than 2)

– Also includes trisomy (having an extra copy of a chromosome; means you’d have 3 copies of a given chromosome rather than 2).

Nondisjunction cont’d

Examples:

Turner Syndrome(XO – monosomy, missing an X chromosome)

Nondisjunction cont’d

• Down Syndrome– Trisomy,

individual receives an extra copy of chromosome 21 (“trisomy 21”).

Nondisjunction cont’d

• Klinefelter Syndrome– A male with two X

chromosomes (XXY) instead of one.

– Still has usual male features, but altered proportions.• Usually taller than

average.

– Individuals with this are usually infertile.

Karyotypes

• A convenient way to look for conditions caused by nondisjunction is to create a karyotype image.– A karyotype is a depiction of

chromosomes as they would appear under the microscope.

–Homologous pairs are usually quite clear; finding extra or missing copies of chromosomes indicates disorder.

Karyotypes - Normal

Karyotypes - Abnormal

Pedigree Charts

• A convenient way to track inherited conditions is through a pedigree chart.

• The chart tracks marriages and offspring produced, gender, and the presence of conditions or disorders.

• Each row (in descending order) represents a new generation in a family.

Pedigree Charts