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!

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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.

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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