Finals Study Guide Chapter 14 Flashcards
Human Variation
• Humans vary in many different ways
• We have talked about three diseases – PKU, cystic fibrosis,
and Tay-Sachs disease – that vary in different populations.
PKU
PKU (metabolic disease in which phenylalanine cannot be
processed and accumulates in the body)
• Most likely occurs in a person of European descent than African or Asian descent
• Same with cystic fibrosis
Tay- Sachs
Tay-sachs disease is concentrated in Jewish populations
Understanding Variation
- Two main sources of variation: Genetic and environmental
- Difficult to determine relative contributions of each
- Genetics and environment both influence phenotypes
- Variation within vs. among groups
- Think of athletes!
Variation in Traits Influenced by Single Genes
• Very few phenotypes are controlled by a single gene. Variation in single genes can explain variation in some phenotypes.
• Genetic variation
- Ex. Sickle-cell anemia (reduces a person’s ability to transport oxygen through the body)
- Another ex: Specific language impairment (SLI)
Why do diseases like PKU, sickle-cell anemia, and cystic fibrosis, which are caused by recessive genes, continue to persist in the population at all?
This happens especially when an allele is rare, and the disease occurs only with a homozygous recessive genotype
Think of it as hidden variation, and there is a selection-mutation balance.
Causes of Genetic Variation Within Groups
In some areas of West Africa, the allele that causes sickle cell exists at a much higher frequency
• WHY? Allele that causes sickle-cell anemia
simultaneously protects the individual against malaria.
Causes of Genetic Variation Within Groups
• Heterozygous individuals can both ward off malaria and
make normal red blood cells
• ¼ of their children homozygous dominant (normal red
blood cells, not resistant to malaria)
• ¼ of their children homozygous recessive (malaria
resistant, but with sickle-cell anemia)
• Because of the fitness advantage the heterozygous
individuals have, the sickle-cell allele remains in the
population.
• This is known as polymorphism.
Genetic Mismatches for Environment
- Non-insulin-dependent diabetes
* Diabetes - cells ignore insulin, which is supposed to promote sugar uptake into these cells. Sugar accumulates in the bloodstream, leading to health problems
* Genetic basis
* Diabetes affects 30% of the population on the Micronesian Island of Nauru
* Same genes allow fat storage to avoid famine
* With a western diet, this genetic profile cannot move sugar into their cells as rapidly
Causes of Genetic Variation Among Groups
- Natural selection in different environments, population subjected to different natural selection pressures
- Hemoglobin allele that causes sickle-cell anemia in homozygous individuals is practically zero in many parts of the world
- There are no mosquitos in the arctic
Genetic Variation: Lactose Tolerance
- Babies produce an enzyme called lactase
- Adults stop making lactase (milk digestion problematic)
- Humans: approx. 5yo
- Europe and north Africa, ability to digest lactose persists
Selective sweep
We know that when a genetic variant is particularly beneficial, it spreads rapidly through the population. This is called selective sweep.
Detecting Selection in the Genome
How much impact does natural selection have?
We know that when a genetic variant is particularly beneficial, it spreads rapidly through the population. This is called a selective sweep.
These chunks of DNA are known as haplotypes. Eventually, these sequences are broken up through recombination, but that takes a while.
Detecting Selection in the Genome
For recent natural selection, we look for long sections of DNA that are nearly identical in populations.
Sometimes these regions differ only very slightly, called single nucleotide polymorphisms (SNP) or “snips”
Genes affecting reproduction, the immune system, skin and eye color, bone development, and digestion have all been under intense selection in the last 10,000 years.
