Download Chapter 2 Human Genetics Overview The purpose of this chapter is

Chapter 2
Human Genetics
Overview
•
The purpose of this chapter is to provide a brief background in molecular and Mendelian genetics necessary for an
understanding of the evolutionary process.
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Genetics can be studied at many levels, with an emphasis is on the concept of information transmission.
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When one concentrates on the cellular level this type of research is labeled molecular genetics.
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It concerns itself with determining what genes are and how they act to produce biological structures.
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In particular the structures of RNA (ribonucleic acid) and DNA (deoxyribonucleic acid) are arenas of study.
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Mendelian genetics is approached in a similar fashion, emphasizing basic principles of inheritance.
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The third area of genetic research discussed in this book is population genetics; this is research that determines changes in
the frequency of genes and DNA sequences in populations over time.
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Microevolution is the changes that take place in the frequency of genes within a population.
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Macroevolution is the projection of these findings to better understand long-term patterns of evolution over time, and
the origin of new species
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An optional primer on cell biology is provided in an appendix at the end of the book. I urge all students to review (or study)
the material presented in the appendix.
Molecular Genetics 1
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DNA: The genetic code
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The DNA molecule provides the codes for biological structures and the means to translate this code (codes for
proteins).
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DNA provides information for building, operating, and repairing biological organisms.
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The structure of DNA
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DNA consists of two strands arranged in a helix joined together by chemical bases.
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The DNA molecule resembles a ladder twisted in a helix shape.
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Ladder rungs represent chemical units called bases (also called nitrogenous
bases).
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There are four possible types of bases: A (adenine), T (thymine), G
(guanine), and C (cytosine). Chemical bonds: A is complementary to T
while G is complementary to C

One of the differences between DNA and RNA is in one of these bases.
In RNA, there is a replacement of thymine with Uracil (U).
The two sugars
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The backbone is made of a sugar-phosphate structure.
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It is to this structure that the bases are attached.

Again it is in this structure that a difference between DNA and RNA is seen
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The sugar in DNA is called deoxyribose (meaning a lack of an oxygen; it is a variant of ribose (a 5-carbon sugar)
Molecular Genetics 2
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The structure of DNA (continued)
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DNA codes for proteins (which are strains of amino acids) using the 4 different bases.
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There are 20 amino acids found in humans
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As the DNA is grouped into triplets (such as ATC or ATT) this means there are 64 (43) triplets!

Think duplicates for some amino acids (64 codons, but only 20 amino acids).

These triples are called codons (see Table 2.1 for the DNA version. In fact, Table 2.1 is unusual display, most
tables give us the mRNA version)
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Functions of DNA
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The DNA molecule can make copies of itself. This process is called replication.
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A second process undertaken by DNA is protein synthesis

The DNA never leaves the nucleus, but the location of protein synthesis is in another part of the cell (the
ribosome).

So there is a set of other nucleic acids, called RNA (ribonucleic acid), that carry out the actual production process.

The RNA (ribonucleic acid) molecule serves as the messenger and decoder for the information in the DNA
molecule.

The ability of the DNA molecule to control protein synthesis involves the attraction of the complementary
bases.
Relethford Chapter 2 Page 1
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The active strand attracts the free-floating bases to form a messenger RNA (mRNA).
Diagram that Shows Translation & Transcription
Not pictured
Molecular Genetics 3
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Functions of DNA (continued)
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Protein synthesis (continued)

This mRNA transfers its information (the free-floating bases) to a second RNA called transfer RNA (tRNA). This
is called transcription.

This is yet another free-floating molecule that does it work in an organelle called the ribosome.

The tRNA brings amino acids to the ribosome (the combined tRNA and its amino acid is called an aminoacyltRNA)
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Your book does not mention this, but there is actually yet a third type of RNA. This is called ribosomal RNA (rRNA).

The rRNA is not free-floating, but is located inside the ribosome; actually it consists of two structures, one larger
and one smaller.
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The tRNA and the rRNA work together to create the long chains of proteins by placing each amino acid into the
correct position. This process is called translation.

Several ribosomes may come together to translate the mRNA simultaneously.
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Chromosomes and genes
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The nuclear DNA is bound together in long strands called chromosomes.
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The number of chromosomes varies by species. Humans have 46 (23 pairs) while chimpanzees and gorillas have 48 (24
pairs)
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About 10 million years ago human ancestral DNA fused two chromosomes together into our modern chromosome 2.
Karyotype of Human Chromosomes
Not pictured
Molecular Genetics 4
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Chromosomes and genes
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There was a time scientists thought there were over 100,000 genes given that is the number of proteins identified.
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In 2003, they discovered there are only about 23,000 genes!
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Now the estimate is even lower, around 21,800. This suggests DNA is a more complex structure that previously
thought
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The chromosomes house the genes.
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Gene refers to a section of DNA that has an identifiable function, such as the gene that determines a particular
blood group
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The location of a particular gene is called a locus (plural, loci)
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Some genes code for proteins, other genes “act as switches”.
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Most of the DNA is not expressed during protein synthesis.
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The parts that are translated are called exons.
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The other parts are called introns (intragenic region). Sometimes introns are called nonsense DNA or junk
DNA; this is not very accurate.
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New research has determined introns are very important regions. They help regulate the genes, as do what are
called the regulatory genes.
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Regulatory genes have a significant effect on DNA production.
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The most important of the regulatory genes are called homeobox genes (Hox genes).
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These genes determine the basic “design” of organisms, such that they segment the body and otherwise guide
the overall body structure.
Molecular Genetics 5
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The Human Genome Project was an international effort, partly non-profit, partly for profit began in 1990, but was planned
in the early 1980s.
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I remember discussion of sequencing the human genome in the early 1980s as a biology major.
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My professor estimated it would take 50 years, given the technology available then
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Figure 2001 as the ‘finish date’ and you guessed right that new techniques were developed.
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Here is a timelineof accomplishments
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The HGP announced the completion in 2000, but not really completed. Announced as completed (again) in 2003.
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Among the interesting findings was that only about 1.5% of DNA codes for proteins.
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The results are also useful for biological anthropologists in comparisons between species.
In addition to the discovery that “junk DNA” is not all junk there has been work in epigenetics.
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Which genes are expressed in different cells (cell differentiation) is a focus of the study of epigenetics (the non-genetic
influences on gene expression).
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DNA is wrapped around proteins and the genes are controlled by genetic “tags” called the epigenome.
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The environment can alter which tags are active.
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They can continue across several cell divisions and can be inherited across generations
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Molecular Genetics 6
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Mitosis and meiosis
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There are two types of chromosomes:
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Autosomal chromosomes (number 1-22) are the non-sex chromosomes
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Sex chromosomes are the X and Y chromosomes.
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If one has 2 XX chromosomes she is ‘female’
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If one has 1 X chromosome and 1 Y chromosome, he is ‘male’
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Mitosis is the duplication of somatic cells (all non-gamete cells). Each cell produces two identical copies.
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Soma cells in humans have 46 chromosomes
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This results from having 2 copies of each chromosome (23 pairs)
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Here is an animation website: http://www.csuchico.edu/~jbell/Biol207/animations/mitosis.html
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Meiosis is the development of sex cells through a separate process. This is how one inherits the chromosomes from
one’s parents.
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Need to reduce the number of chromosomes to 23 (one from each pair of chromosomes)
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Do this and then ‘package up special’: into eggs and sperm for humans
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Here is the animation: http://www.csuchico.edu/~jbell/Biol207/animations/meiosis.html
Mitosis and meiosis pictures not included in the notes; see text, please.
Mendelian Genetics 1
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Austrian priest Gregor Mendel (1822-1884) carried out an extensive series of experiments in plant breeding, using pea
plants. Mendel explored 7 traits of peas
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His carefully tabulated results provided the basis of what we know about the mechanisms of genetic inheritance.
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Mendel’s experiments demonstrated a pattern of inheritance unknown before. He showed that genetic information
is inherited in discrete units (genes).
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Mendel showed that genes do not blend together in an offspring.
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It took until the 1950s to have the specific structure of inheritance identified and described (DNA/RNA) by Watson
and Crick (Mostly based on the uncredited work of Rosalind Franklin).
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The DNA molecule provides the codes for biological structures and the means to translate this code.

DNA provides information for building, operating, and repairing organisms.
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Genotypes and phenotypes
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The actual code for a specific gene is called its genotype
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The variants are called alleles. Sometimes an allele is expressed by making a different version of the protein or
blocks the making of a protein, and so on
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For some genes, there is more than one version in their coding. This condition is called polymorphism (meaning
many shapes)
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The term for what the organism looks like as a result of this gene (or its interaction with other genes) is called its
phenotype.
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The phenotype is also affected by the environment.
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The relationship between genotype and phenotype is affected by the relationship between the two alleles
present at any locus.
Mendel’s Pea Pods
Not pictured.
The seven phenotypic characteristics investigated by Gregor Mendel in his experiments on breeding in pea plants. Each of the
seven traits has two distinct phenotypes.
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Mendelian Genetics 2
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Mendel’s law of segregation
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He deciphered two laws; one of them was the Mendel’s law of segregation: Sex cells contain one of each pair of alleles.
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When a person receives two different versions of the gene on their two chromosomes, this is called heterozygosity
(they are heterozygotes).
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If the person receives two genes that are the same this is homozygosity (they are homozygotes).
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Of course the exception is with the sex chromosomes of human males.
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They only have 1 copy of the X and one of the Y.
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This is why you often hear of X-linked traits or Y-linked traits.
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Dominant and recessive alleles
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In a heterozygote (two alleles at a given locus are different), an allele is dominant when it masks the effect of the other
allele at a given locus.
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A recessive allele is one whose effect may have been masked.
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In a homozygote, both alleles at a given locus are identical.
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What dominance is NOT:
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This does not mean strong versus weak. Blue-eyed humans are not weaker, for instance.
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This does not mean more common and less common. Dominant allele may be very rare.
Model of A Dominant/Recessive Trait
Not pictured
Mendelian Genetics 3
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Dominant and recessive alleles (continued)
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Nomenclature.
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Except for a few exceptions (such as the ABO blood group), the dominant gene is capitalized and the recessive is
lower case
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Example: T is a taster gene and is dominant, t is the non-taster version and is recessive.
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Sometimes the variants make no difference in how the protein is made (not expressed) and this is called a silent
mutation. An example of dominance is the ability to taste PTC
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If one has 1 (Tt) or both genes for tasting (TT), you do
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If you have no genes for tasting (tt), you do not (I am a non-taster)
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Codominant alleles
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Alleles are codominant when they both affect the phenotype of a heterozygous genotype. Neither is dominant over
the other.
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One example of this is the MN blood group.
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If you are heterozygote, your blood type is also MN (both genes expressed).
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In complex genetic systems with more than two alleles, some alleles may be dominant and some may be
codominant.
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The classic example of this is the ABO blood group
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Both A and B are dominant to O, but A is co-dominant with B
Predicting Offspring Distributions
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When parents each contribute a sex cell, they are passing on only one allele at each locus to
their offspring.
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The possible genotypes and phenotypes of the offspring reflect a 50% chance of transmittal
for any given allele of a parent.
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In the MN blood group, they are co-dominant, which means the genotypes are the
same as the phenotypes
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In the general population the possible genotypes are MM, MN, and NN, but how can
we determine the combinations for just 2 parents? The Punnett square.
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For this example, note that the phenotypes of the offspring could be 25% MM,
50% MN, and 25% NN.
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The percentages change with parents of different genotypes.
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Also, the probabilities of a specific genotype remain the same for each offspring; statistically this is called
selection with replacement (the idea that the egg and sperm make more than one copy of themselves)
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Try a few examples as you will be asked about them on the test.
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Chromosomes and Inheritance 1
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Mendel’s second law
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Mendel did not know about chromosomes, but he did generate a second law based on his pea pod experiments.
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Mendel’s law of independent assortment states that the segregation of any pair of chromosomes does not influence
the segregation of any other pair of chromosomes.
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Confused? I urge you to create your own genome of 3 pairs of cards (aces, twos and threes). Only use two suits.
Then practice.
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The exception to this law is linkage. Linkages are alleles found on the same chromosome that are inherited together.
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While loci that are linked tend to be inherited together, there is a way to “break up” the linkages: recombination.
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Recombination is the production of new combinations of DNA sequences caused by exchanges of DNA during
meiosis.
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Crossing over is the actual exchange of DNA between chromosomes during meiosis.
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Sex chromosomes and sex determination
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An individual’s sex is determined by one of the 23 pairs of human chromosomes, the sex chromosome pair, X and Y.
Females have two X chromosomes (XX) and males have one X and one Y chromosome (XY).
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Due to this difference, some recessive conditions are more common in men than women: hemophilia and red-green
color blindness for instance.
Chromosomes and Inheritance 2
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Inheritance from one parent
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Mitochondria (singular, mitochondrion) are structures contained within the
cytoplasm of eukaryotic cells that convert energy, derived from nutrients, into a
form that is used by the cell.
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Mitochondrial DNA (mtDNA) directs the conversion of energy within cells
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Mitochondrial DNA is DNA found in the mitochondria that is inherited only
through the maternal line.
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This means that neither meiosis nor recombination occur
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All variants are the result of mutation only.
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It has a circular structure and has about 40 genes.
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Mitochondrial DNA has the same molecular structure and function as nuclear
DNA.
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mtDNA is used to investigate
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Evolutionary relationships between species
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To trace ancestral relationships within the human lineage
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To study genetic variability among individuals and/or populations.
Diagram showing the genes found
on mtDNA
The Genetics of Complex Traits 1
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Complex traits are characteristics of interest in human evolution, and include: skin color, body size, brain size, and
intelligence.
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These are not simple, discrete traits.
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Complex traits have a complex mode of inheritance.
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One or more genes may contribute to the phenotype.
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They may also be affected by the environment.
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The combined action of genetics and environment produces traits with a continuous distribution.
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Polygenic traits and pleiotropy
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Polygeny and pleiotropy explain the combination of interactions that can occur if traits are complex.
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Polygenic traits are traits resulting from two or more loci. When several loci act to control a trait, many different
genotypes and phenotypes can result.
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Pleiotropy occurs when an allele effects more than one trait.
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Both polygeny and pleiotropy can occur at the same time.
The Genetics of Complex Traits 2
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Nature versus nurture
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This is a long-standing debate this itself is more complex than once thought.
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Really the question is no longer one or the other, but how much each contributes in a specific situation.
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We now acknowledge that both genes and environment have an effect on both biology and behavior.
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Heritability is the proportion of total variance in a trait that is attributable to genetic variation in a specific population.
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Formulae:
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Total variation = genetic variation + environmental variation
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Heritability = (genetic variation/total variation)
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A high value means more contribution of genetics.
Points to remember:
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Heritability can differ between populations.
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It can change in the same population over time
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Because it is measured at the population level it is not applicable to an individual’s phenotype.
Mutations 1
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Evolutionary significance of mutations
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Mutations are changes in the genetic code and the ultimate source of all genetic variation.
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Mutations are caused by a number of environmental factors such as background radiation, heat, and ingested
substances such as caffeine.
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Mutations can take place in any cell of the body. To have evolutionary importance, the mutation must occur in a
sex cell.
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Mutations are random. There is no way of predicting when they will take place.
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Mutations can have different effects, depending on the specific type of mutation and the environment
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Mutations are sometimes advantageous, leading to changes to improve the survival and reproduction of organisms.
Sometimes they are deleterious. Often they are neutral (silent mutations).
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Rates of mutations
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Specific mutations are relatively rare events.
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The exact rate of mutations is difficult to determine. Estimated range in the rates of mutations in humans: 0.1 to 10
mutations per million genes per generation.
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Mutations are more apparent if they involve a dominant allele.
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Harmful mutations may result in spontaneous abortion before a pregnancy is detected.

Mutation rates are generally low. More mutations in sperm than in eggs (think of how these are produced and in
what quantities).
Mutations 2
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Types of mutations
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Mutations can involve changes in a single DNA base, larger sections of DNA or an entire chromosome.

Changes in a single DNA base

This is often called a point mutation

Sickle cell anemia is an example of the substitution of one DNA base for another.

Changes in larger sections DNA

Genetic information contained in chromosomes can be altered by deletion or duplication of part of a chromosome.

Crossing-over is one mechanism that results in this. Parts of chromosomes can break off and be reattached
backwards, also.

Changes in an entire chromosome, including deletion or duplication of the entire chromosome.

Genetic information contained in chromosomes can be altered by deletion or duplication of part or all of the
chromosome.

Monosomy is a condition in which one chromosome rather than a pair is present in body cells.

Trisomy is a condition in which three chromosomes rather than a pair occur in body cells.
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The most important evolutionary point to keep in mind about mutations is that they are the ultimate source of all
variation (more on this is Chapter 3).
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