Genetics and Evolution

Question 1

Genes

Answer 1

• A sequence of nucleotides on a chromosome that carries the information for a specific protein, the basic unit of heredity.
• Locus = position of a gene on a chromosome
• For most genes, all members of the same species will have the same nucleotides in the same location on the same chromosome.

Question 2

Alleles

Answer 2

• The variants of a gene resulting in differences in the expressed protein and therefore the expressed trait.
• Blood types come from a gene with three alleles.
  
  • A,B and O → each codes a different version of a maker protein on the red blood cell
    
    • However, each individual only has two, one from each parent

Question 3

Genotype

Answer 3

• The specific alleles for a gene that an organism carries

Question 4

Homozygote

Answer 4

• A description of the gene for an individual who carries two of the same allele for a gene.
• If a person is homozygous for a trait that means that both alleles are the same

Question 5

Heterozygote

Answer 5

• A description of the genotype for an individual who carries two different alleles for a gene
  
  • Heterozygous = different alleles

Question 6

Hemizygote

Answer 6

• A description of the genotype for an individual who carries only one gene for a trait.
• Hemi means “half”
  
  • This can occur because a gene is accidentally deleted or because maybe because an entire chromosome is lost
• The most common instance in humans is the sex chromosome in typical genetic males.
  
  • Recall that the sex chromosome in humans are called X and Y.
  • Males have an X and Y chromosome, while females have only X.
  • Thus, any gene for a male, carried on their X or Y chromosome is unpaired and is called hemizygous

Question 7

Phenotype

Answer 7

• The physical trait resulting from the genes due to the proteins produced from them.

Question 8

Dominant Gene

Answer 8

• The allele is always expressed when present

Question 9

Dominant Gene

Answer 9

• The allele is always expressed when present

Question 10

Recessive Gene

Answer 10

The allele is only expressed in the absence of another allele

Question 11

Complete Dominance

Answer 11

• The presence of the dominant allele hides the presence of other in a heterozygote.
• Complete dominance in humans is very hard to find, but the clearest examples come in the form of genetic disorders, like Huntington disease, where the disease - causing mutation to the huntingtin protein is dominant to the normal version.

Question 12

Codominance

Answer 12

• The inheritance pattern demonstrated when both characteristics of the different alleles are observed in a heterozygote.
• Blood types A and B are a perfect example, as a person with AB blood has both proteins
  
  • A and B are codomiant relative to each other, but A and B are completely dominant relative to the O allele.

Question 13

Incomplete Dominance

Answer 13

• The inheritance pattern demonstrated when a bleeding of the characteristics of the different alleles result in a phenotype between the phenotype caused by either allele alone.
• A common example in humans is when curly hair is mixed with straight hair with produces wavy hair.
  
  • This characteristic is actually controlled by a single gene. → KRT 71 gene on chromosome 12 which encodes the protein keratin in hair.
  • A mutated version of this genes adds extra cystine residues in the protein which causes cross linkages which lead to curly hair. When a person inherits both a curly and a straight allele they simply express fewer of the mutated proteins leading to the wavy hair phenotype not curly, not straight.

Question 14

Penetrance

Answer 14

• A description of the rate at which a trait is expressed.
• Complete penetrance would mean that 100% of those with a certain genotype will express the associated phenotypes.
  
  • In reality, things are rarely 100%
• For example, a certain mutation in the BRCA2 gene is associated with the risk of breast cancer, but does not always lead to breast cancer 100% of the time. Instead this mutation is 60% penetrant, meaning that out of 100 patients with this mutation on average 60 will develop breast cancer, but not all of them.
  
  • All 100 patient share the same genotype, yet not all of those genotypes relate in the same phenotype

Question 15

Expressivity

Answer 15

• A description of the degree to which a gene is demonstrated by an individual.
• Two individuals might have the exact same genes, but they can express those genes at different amounts.
• Marfan syndrome is an autosomal dominant disorder that is a great example of variable expressivity.
  
  • It is nearly 100% penetrant, meaning that almost everyone with the gene expresses the syndrome
  • However, not all individuals with the condition exhibit the symptoms to same degree
  • This variation in severity illustrates variable expressivity of the trait

Question 16

Mutations

Answer 16

• Can be categorized into three major classes:
  
  • Point mutation caused by alterations in a single nucleotide
  • Frame shift mutations, which change the reading frame of the DNA sequence due to an insertion or deletion of nucleotides
  • Chromosomal which involved complete deletion, duplication, inversions, insertions, or translocations of larger segments of DNA

Question 17

Point Mutations

Answer 17

• Can be categorized as silent mutations, where the change in nucleotide has no effect on the final phenotype.

Question 18

Missense Mutations

Answer 18

When the change in nucleotide results in the substitution of one amino acid for another coded in the final product

Question 19

Nonsense Mutations

Answer 19

When the change in nucleotide results in a stop codon

Question 20

Segregation of Genes

Answer 20

• Each gene has two variants or alleles.
• An allele is the variants of a gene resulting in differences in the expressed protein and therefor the expressed trait.

Question 21

Law of Segregation

Answer 21

The two alleles segregated during meiosis, resulting in gametes that carry only one allele for any inherited trait.

Question 21

Law of Segregation

Answer 21

The two alleles segregated during meiosis, resulting in gametes that carry only one allele for any inherited trait.

Question 22

Law of Independent Assortment

Answer 22

It states that the inheritance of one gene does not affect the inheritance of another gene.

Question 23

Polyploidy

Answer 23

• Cells of the organism contain more than two sets of chromosomes
• In humans and other animals, polyploidy is fatal.
• To resolve this, germ cells undergo a different type of cell division than mitotic cell division.
  
  • germ cells undergo meiosis in which their cells are called haploid cells
    
    • The zygote formed will be a diploid cells with a complete set of chromosomes.
    • Only half of the genetic material from the gametes are coming from mom and half are coming from dad, this process of meiosis opens the door for potential genetic variability

Question 24

Stages of Meiosis

Answer 24

1. Prophase - Preparation
2. Metaphase - Middle
3. Anaphase - Action
4. Telophase - Termination
   
   1. Cytokinesis - cleavage

• Unlike mitosis, this division cycle is carried out twice. Thus, instead of producing two daughter cells there are four daughter cells called gametes.

Question 25

Meiosis

Answer 25

• An important process unique to meiosis I is synapsis.
  
  • Two chromosomes join together in joining called tetrad, which allow gene combinations
  • To connect the two chromosomes of a tetrad, a group of proteins called the synaptonemal complex is needed
  • This connection holds chromosomes together so that crossing over can occur.
  • The juncture at which the synaptonemal complex forms is called a chiasma.
  • Crossing over facilitates the exchange of DNA segments and is responsible for genetic variation.
    
    • This is why children are similar to their parents but still have unique traits.
  • After any crossing over of genes were left with recombinant chromosomes
    
    • Chromosomes can crossover at more than one point along their length, which can make recombinant DNA very different from the original chromosome.
    • The tetrads remain connected as the cell moves to metaphase I
  • The seperation of chromosomes in Anaphase I is known as disjunction and it randomly sorts the maternal and paternal chromosomes in different daughter cells. This is called segregation.

Question 26

Nondisjuctions

Answer 26

• When the homologous chromosomes do not separate. This can lead to daughter cells with the incorrect number of chromosomes
• Nondisjunction can lead to gametes that cannot produce viable offsprings or produce offsprings with genetic disorders
• Note that recombination does not occur in meiosis 2