Genetics and Biodiversity Notes

Polygenic Traits and High Cholesterol

• High cholesterol is influenced by multiple genes (at least 3 identified).

• Individuals may have 1, 2, or 3 mutated genes contributing to high cholesterol.

• Polygenic traits are linked to many genes and exhibit a wide range of phenotypes due to gene interactions.

• The phenotype depends on the genotypes of multiple genes, varying among individuals.

• Predicting inheritance of polygenic traits is difficult due to multiple gene interactions.

• For single-gene traits, inheritance can be predicted if parental genotypes are known. Each genotype consists of two alleles located on chromosomes inherited from each parent.

Dominant and Recessive Alleles: Cystic Fibrosis Example

• Cystic fibrosis is linked to a membrane protein, with a dominant allele (F) for the correctly shaped protein and a recessive allele (f) for the mutated protein.

• The dominant allele masks the recessive allele when both are present.

• Genotypes:

  • Homozygous dominant (FF): normal phenotype (no cystic fibrosis).

  • Homozygous recessive (ff): cystic fibrosis.

  • Heterozygous (Ff): normal phenotype (dominant allele masks recessive allele).

• A Punnett square predicts the probability of inheritance.

• Example: Ff x FF cross results in no chance of cystic fibrosis.

• Example: Ff x Ff cross results in 75% chance of no cystic fibrosis, 25% chance of cystic fibrosis.

Incomplete Dominance: Tay-Sachs Disease Example

• Incomplete dominance occurs when one allele cannot completely mask the other, resulting in an intermediate phenotype.

• Tay-Sachs disease involves the ability to break down lipids by a specific enzyme.

• Alleles:

  • EE: normal enzyme production

  • E’E’: no enzyme production (Tay-Sachs disease)

  • EE’: intermediate enzyme production (half the normal amount)

• Individuals with the intermediate phenotype (EE’) do not have Tay-Sachs because the enzyme produced by the one normal allele is sufficient.

• Example: EE’ x EE’ cross.

  • 75% chance of no Tay-Sachs.

  • 50% chance of producing 50% of the enzyme.

  • 25% chance of producing no enzyme and having Tay-Sachs.

Codominance and Multiple Alleles: Blood Type Example

• Human blood type is determined by proteins (antigens) on red blood cells.

• Three possible alleles: IA, IB, and i.

  • Type A blood: “A” proteins (IAIA or IAi).

  • Type B blood: “B” proteins (IBIB or IBi).

  • Type O blood: no proteins (ii).

  • Type AB blood: both “A” and “B” proteins (IAIB).

• IA and IB are dominant, i is recessive.

• Blood type is an example of multiple allelic inheritance, but each individual inherits only two alleles.

• In AB blood type, both alleles are equally expressed, making it an example of codominance.

• Example: AB x O cross. 50% chance of Type A blood (IAi) and 50% chance of Type B blood (IBi). There is no chance of type AB or O blood.

Sex-Linkage and Hemophilia

• Human cells have 46 chromosomes or 23 pairs, with the 23rd pair being sex chromosomes (XX for females, XY for males).

• Genes on sex chromosomes are sex-linked traits (specifically on the X chromosome).

• Females have two X chromosomes, males have one X and one Y.

• Hemophilia is a sex-linked disorder where individuals do not produce enough blood-clotting factors.

• Normal protein production is dominant (XH) and mutated version is recessive (Xh).

• Alleles are shown as being carried on the X chromosome.

• Genotypes:

  • Females: XHXH (normal), XHXh (carrier), XhXh ( hemophiliac).

  • Males: XHY (normal), XhY ( hemophiliac).

• Example: A carrier mother (XHXh) and a normal father (XHY) have a son.

  • 50% chance that the son will inherit hemophilia (XhY).

• Males have a higher probability of expressing sex-linked traits because they only have one X chromosome.

Pedigrees

• A pedigree is a chart showing the inheritance of a trait in a family, using symbols to represent individuals.

• Pedigrees help determine the pattern of inheritance and genotypes of individuals.

• Example: A pedigree showing Cystic Fibrosis inheritance. If parents without the trait have a child with the trait, the trait is recessive and both parents are heterozygous carriers.

Sexual Reproduction

• Sexual reproduction involves two parents contributing genetic material and offspring are genetic combinations of both parents.

• Genetic material is passed via gametes (sperm and egg).

• Fertilization combines genetic material from male and female gametes.

Nondisjunction and Genetic Variation

• Nondisjunction results in chromosomal conditions like Down syndrome (trisomy 21), where there is an extra copy of chromosome 21.

• Chromosomal conditions affect all genes on the affected chromosome.

• Karyotypes visualize chromosomes, but mutations causing gene disorders occur at a molecular level and can not be seen on it.

• Genetic variation describes differences in DNA among individuals.

• These differences lead to variation in the expression of traits.

• Genetic variation allows for favorable traits that increase reproductive advantage.

• In changing environments, these favorable variations are selected and passed to offspring through natural selection, facilitating adaptation and evolution.

Independent Assortment

• Homologous pairs (one maternal, one paternal chromosome) line up randomly during meiosis I.

• Human cells have 23 chromosome pairs, resulting in over 8 million possible arrangements during meiosis.

• This random assortment leads to unique gametes with different combinations of maternal and paternal chromosomes.

Crossing Over

• Genes on the same chromosome are linked and usually inherited together.

• During meiosis, homologous chromosomes can exchange segments (crossing over).

• Crossing over changes the combination of alleles linked on a chromosome.

• This process increases genetic variation.

Biodiversity

• Biodiversity refers to the variety of living organisms on Earth and genetic differences within species.

• Organisms are classified into groups based on similarities and classifications evolve as new scientific discoveries are made.

• DNA technology has allowed scientists to compare DNA sequences of different species to determine relationships.

Taxonomy

• Taxonomy is the science of classifying organisms.

• Living organisms are classified into six kingdoms: Archaebacteria, Eubacteria, Protista, Fungi, Plantae, and Animalia.

• There are 7 levels of organization (taxa): Kingdom, Phylum, Class, Order, Family, Genus, Species.

• Organisms sharing more taxa have more characteristics in common, indicating a closer evolutionary relationship.

• All living organisms have a two-word scientific name (binomial nomenclature) in Latin.

  • Genus (capitalized) indicates a close evolutionary relationship.

  • Species (lowercase) is unique to each organism.

• Occasionally, species are renamed to share the same species name but are given different subspecies names if new scientific information reveals closer relationships.

Cladograms and Phylogenetic Trees

• Cladograms and Phylogenetic Trees are branching models showing relationships among organisms.

• Cladograms are based on shared traits or genetic information and do not indicate time or amount of difference.

  • Organisms at the ends of lines are descendants of a common ancestor, represented at the base.

  • Nodes represent common ancestors.

  • More shared traits indicate closer evolutionary relationships.

• Phylogenetic trees indicate time spans between branching points, representing time since common ancestry.

Dichotomous Keys

• A dichotomous key identifies previously categorized and named species.

• Unique characteristics distinguish each species.

• Keys use paired statements about the presence or absence of characteristics.

• To use, start with the first paired statement.

Example Dichotomous Key:

1a. Organism is a carnivore → go to 2

1b. Organism is not a carnivore → Deer

2a. Organism is in the cat family → go to 3

2b. Organism is not in the cat family → Wolf

3a. Organism has retractable claws → go to 4

3b. Organism does not have retractable claws → Cheetah

4a. Organism has the ability to roar → Lion

4b. Organism does not have the ability to roar → Domestic cat