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