Mendelian Genetics: Chapter 17 Traits

• Chapter 17 focuses on Mendelian Genetics.

Introduction to Traits

• Most traits are controlled by multiple genes in more than one location.
• Baldness is a dominant trait that results in the presence of hair loss, while the absence of baldness is recessive.
• A wavy hair texture is dominant, whereas not wavy is recessive.
• A widow's peak hairline is dominant, and a straight hairline is recessive.
• A cleft chin is dominant, and the absence of a cleft chin is recessive.
• Dimples are dominant, and the absence of dimples is recessive.
• An oval face shape is dominant, and a round face shape is recessive.
• Freckles are dominant, and the absence of freckles is recessive.
• Free earlobes are dominant, while attached earlobes are recessive.
• Long ear length is dominant, and short ear length is recessive.
• Short eyelashes are dominant, while long eyelashes are recessive.
• Normal or perfect eyesight is dominant over nearsightedness.
• Color blindness is a recessive trait.
• The absence of a Mongolian eye fold is dominant, while its presence is recessive.
• A broad nose is dominant, while a narrow nose is recessive.
• A Roman nose is dominant, and the absence of a prominent bridge is recessive.
• A bent thumb is dominant, and a straight thumb is recessive.
• The ability to place the left thumb over the right is dominant, while the opposite is recessive.
• Right-handedness is dominant, while left-handedness is recessive.
• A hitchhiker's thumb is dominant, while a straight thumb is recessive.
• The presence of mid-digital hair is dominant, while its absence is recessive.
• Having six fingers is dominant, while having the normal five is recessive.
• The ability to thumb-up with the right thumb is dominant, while the left thumb is recessive.
• Vulcan fingers are normal, while Earthling fingers are considered recessive.
• Webbed fingers are dominant, and normal fingers are recessive.
• Broad lips are dominant, and thin lips are recessive.
• The ability to roll the tongue is dominant, whereas not being able to roll the tongue is recessive.
• Abundant body hair is dominant, and scant body hair is recessive.
• Normal height is dominant, while dwarfism is recessive.
• Good intelligence in math/science is dominant over good intelligence in art/music.
• Black/brown skin color is dominant over light skin color.
• A, B, and AB blood types are dominant, while O blood type is recessive.
• High blood pressure is dominant over low blood pressure.
• The presence of the Rh factor in blood (Rh+) is dominant, while its absence (Rh-) is recessive.
• A, B, and AB blood types are codominant traits.

Selective Breeding

• Traits have been noticed to be able to be passed down for generations.
• Farmers can select for desirable traits from hearty crops.
• The same applies to domesticating animals for farm work.
• With understanding genetics, it is possible to select for desirable traits and behaviors, shown with new species like Canola.

Genes

• Genes are segments of DNA that determine specific inheritable traits like eye color.

Alleles

• Alleles are the different variations of genes on the chromosome, influencing expression.
• A blue eye allele contrasted to brown eye allele is an example, where two genes are alleles.
• One allele for every gene is inherited from each parent, resulting in two alleles per gene -- one from each parent (diploid).
• What is expressed depends on how those alleles are paired up, and if the traits are dominant or recessive (H vs. h).

Genotype vs. Phenotype

• Genotype: Combination of alleles that encodes a given trait or sequence that makes phenotype.
• Phenotype: The physical expression of the genotype that is observed.

Allele Combinations

• Traits are influenced by the interaction of two alleles, with one from each parent.
• Homozygous Dominant HH: A dominant allele from each parent, resulting in a dominant phenotype.
• Heterozygous Hh: A dominant allele from one parent and a recessive allele from the other, resulting in a dominant phenotype.
• Homozygous Recessive hh: Recessive alleles are from both parents, resulting in a recessive phenotype. The recessive trait is only expressed this way.

Mendel's Experiments Pt. 1

• True breeding crosses (homozygous both traits).
• P1: tall plant x tall plant = 100% tall offspring.
• P1: short plant x short plant = 100% short offspring.
• Then bred.
• P2: tall x short plant = F1 100% tall plants.

Mendel's Experiments Pt. 2

• F1 was crossed of two tall plants.
• Results: F2 mainly tall but, some short offspring.
• The offspring displayed a parental phenotype rather than a blend of traits, shown as a dominant vs. recessive characteristic.
• The F1 generation appeared tall, but they inherited a short allele that was "masked" due to the other allele's dominance.
• Short phenotype only seen when paired with a second recessive allele.

Mendel's Other Pea Traits

• You are not responsible for knowing these, but being aware that each trait is controlled by a gene.
• Each gene has an allele, and it takes two alleles (one from each parent) to create the genotype to express the phenotype.

Mendel Crosses

• P refers to the Parent generation.
• F1 refers to the First generation offspring from the parent generation.
• F2 refers to the Second generation offspring, done by crossing two F1 individuals.
• As long as we know the genotype of parents being crossed, we can determine the ratio and potential offspring genotypes through a Punnett square.
• We must know the mode of inheritance! (autosomal dominant or recessive).
• Autosomal refers to any chromosome except X/Y.

Genetic Laws

• Law of Independent Assortment: Each allele pair will segregate independently of each other during sex cell formation.
• Law of Segregation: Allele pairs will separate in sex cell formation. This is gamete formation, where homologous pairs are separated into different cells.

Monohybrid Crosses

• Consider only one trait.
• Black fur is dominant to brown fur in Mice, as an example.
• List the genotypes of a black and a brown mouse.
• If a heterozygous black mouse is crossed with a brown mouse, determine the probability that their offspring will have black or brown fur.
• If two heterozygous black mice are crossed, what will be the genotypic and phenotypic ratio of their offspring?
• Is it possible for a homozygous black mouse and a heterozygous black mouse to have "grandchildren" that are brown?

Test Crosses

• Utilized when the genotype of the offspring is known, but the goal is to find the proper parent genotype combination.
• This is done by crossing the unknown individual with homozygous recessive.
• If dominant can be HH or Hh.
• 100% of offspring are dark means the dominant is HH.
• 50% of offspring are dark means the dominant is Hh and are a carrier for recessive.
• 100% of the offspring are light means hh.
• Getting the offspring ratio should tell which dominant genotype it is!

Dihybrid Crosses

• Investigates the inheritance of two different genes and their alleles at once. Example is Seed color and seed shape RrYy.
• Find Gametes by FOIL.
• RrYy = RY, Ry, rY, ry (remember independent assortment?-- Chromosomes line up randomly - so all gamete possibilities!)
• P1 is two true breeding
• All of F1 is the dominant trait.
• F2 is a 9:3:3:1 ratio of: DD, DR, RD, and RR -- only if mating two double heterozygotes.

Solving Genetic Problems - Pt. 1

• ID parent genotypes
• Identify what are the dominant and recessive alleles in the question
• List them out
• ID possible gametes
• FOIL for dihybrid crosses
• Don't repeat same gametes-- just make a smaller P square (has same ratios!)

Solving Genetic Problems - Pt. 2

• P Square
• Cross alleles with each other
• Be sure to include one allele for each trait in each column
• ID genotypic AND phenotypic ratios
• Even if asking for phenotype ratio, show the genotype that got that.
• Ratios are in lowest terms.
• If a trait they asked for is not there, then = 0
• More likely decimal than percentage.

Monohybrid & Dihybrid Cross Examples

• Black fur in mice is completely dominant to brown fur. Short tails are completely dominant to long tails.
• A cross is done between a true-breeding black, long-tailed mouse and a heterozygous black, short –tailed mouse.
• Black hair is completely dominant to red hair. Hazel eyes are completely dominant to green eyes. If a black haired, hazel eyed man, heterozygous for both traits has babies with a woman with the same genotypes, what is the phenotypic ratio for their offspring?

Incomplete Dominance

• When both alleles contribute to the phenotype.
• No one allele is completely dominant (like our former examples) to conceal another trait.
• Creates a blended phenotype when heterozygous.
• NOT using capital and lowercase letters.
• Format: single letter (represents the gene) with different superscripts for the alleles.

Incomplete Dominance Example

• Sickle-cell anemia is a form of incomplete dominance where heterozygotes have increased fitness when exposed to malaria.
• Considering a man with normal blood cells having children with a heterozygous woman, what is the probability that their children will be born with severe anemia?
• What proportion of their children will be more likely to survive malaria exposure?

Codominance

• Both alleles are fully expressed at the same time.
• Results in a mixed phenotype.
• Only one copy of each allele is needed to be expressed on both ("dominant" alleles).
• Heterozygous = both alleles appear fully (not a hybrid, but a mix).
• Format: same as incomplete dominance.
• In some varieties of flowering plants, the petal color is determined by two alleles that demonstrate codominance. One version of the allele R1, encodes red flowers, while the other version R2, encodes white flowers.

Blood Types

• Type A and Type B blood exhibit codominance with each other. Ex) Type AB blood; I^A I^B
• Type A and B blood both exhibit complete dominance over Type O. Ex) ii for type O only
• Both uppercase/lowercase format can be seen, as well a superscript when denoting blood type. I^A = I^B > i
• What determines if someone has a positive or negative blood type is Rhesus Factor. Someone will either have the presence, or absence, of an antigen. Rh factor in blood (Rh+) is dominant.
• Ex) A heterozygous man with type B blood has children with a woman who is type O blood (What possible blood types could their children have?) Examples in figures.

Multiple Alleles - Hierarchical Dominance

• Some genes are controlled by multiple alleles, where multiple alleles result from different mutations of a single gene.
• There will be an order of dominance as to what phenotype will be expressed. Ex) A hierarchy: One more dominant than the other is more dominant than the other.

Examples of Multiple Alleles - Hierarchical Dominance

• What is the phenotypic (and genotypic) ratio of offspring when an apricot with genotype E2E4 is crossed with a heterozygous honey-eyed drosophila? (Consult the table for Eye Colour Dominance)

Sex-Linked Traits

• Traits that are carried on the X chromosome and show different inheritance pattern than genes on autosomes. Always never the Y.

• More common in males, this can be x male-pattern baldness, color blindness, and hemophilia.

  • All recessive traits that are carried by the mother that are passed onto the son (as the son's only have the one X!).
  • Females must have two recessive alleles to express this trait themselves. Otherwise the mother can be an unaffected carrier.
  • Sexes MUST be reported in answers (as answers depend on alleles on X chromosomes).

• Sex linked traits and the link to males.

  • Who do males inherit their sex-linked traits from?
  • Can a male be a carrier in this situation like they would for Mendel traits and why/why not?

Sex Linked Examples

• What are the probabilities of gender and being colorblind from a homozygous mother with normal vision and father with colorblindness.
  • A colorblind man and a woman, homozygous for normal vision, have three children: two girls and a boy.
    • What is the probability that both their daughters are both carriers?
    • What is the probability that their son is colorblind? How about their next son? Their next child?

Barr Bodies

• Although females carry two X chromosomes in each of their cells, only one chromosome is active.

• The other one condenses/"shuts off” and is called a Barr Body.

• The particular X chromosome that is inactivated in each cell is completely random and is a form of codominance.

• This is why calico cats are female only! Males will inherit one or the allele from mom. Can't express both. One or the other.

• Since the calico cat gene is codominant and sex-linked, the genotype would look like XBXR for a female calico cat.

• Show the cross of a female calico cat with a black male. - Determine the percentage of kittens that will be: - Black and male - Calico and male - Calico and female

Polygenic Traits

• Many traits are controlled by more than one allele at different loci.
• Phenotypes vary gradually from one extreme to another rather than being one or another, making it a continuous trait. Ex) Skin color and height are examples of polygenic traits.

Epistatic Genes

• Genes that control whether or not another gene is expressed.

• This is a set of alleles that determine whether the alleles in the other location are expressed or not.

  • Two genes involved in Labrador coat color, gene 1 = Coat color where B greater than b. Gene 2 = Color expression where color is either expressed (E) or inhibited (e).
  • Dominant color expression allele (E) must be present in order for the black or brown coat to show.
    • • If the color expression is homozygous recessive (ee), then the typical coat color gene is not expressed = yellow lab.

• Gene examples and the control of sharpness of spines in a type of cactus.

• Cacti with the dominant allele S have sharp spine, whereas homozgyous recessive ss cacti have dull spines.

• At the same time, second gene, N, determines whether cacti have spines. Homozygous recessive nn cacti have no spine.

• If doubly heterozygous SsNn cacti were allowed to self-pollinate, what ratios would be observed to give the offspring to be produced via a cross between a true-breeding sharp spines cactus and a spineless cactus (homozygous recessive for both traits).

Gene Linkage

• Tendency for alleles to be inherited together during meiosis due to how close they are located to each other on a chromosome Genes whose loci are closer together, are less likely to be separated to different chromatids during crossing over. Since close together, then crossing over more likely to occur elsewhere (like between genes that are farther apart) and these two genes will stay together. Doesn't ever never cross over between these genes! Only less likely.

Frequency Distribution

• The relationship between alleles and the frequency we find them.
• Percent Recombination = #recombinant offspring / Total # of offspring x100%
• Non-linked genes = Roughly 50% parental and 50% recombinant genes
• Linked genes = Less than 50% of recombinants/mainly parental phenotypes % Recombination = How many map units apart (distance between the genes are from each other) The recombination frequency lowers which equals to the genes are closer together.

Gene Mapping

• Find the two traits that are farthest away from each other.
• Then find one other trait that is shared between those two traits
• Will help map the sequence of the gene loci on the chromosome
• May have to calculate map units as # recom/ total offspring ex. map the distance from two genes and determine which are closely related, and which are the furthest apart.

Pedigrees and Analysis

• A type of flowchart that uses symbols to trace expression of a particular trait through multiple generations of a family We can interpret them to figure out what type of trait is being shown as well as potentially determine the genotype of family members (once we figure out the type of inheritance)
• Roman numerals = generations
• • Strategy:
    • Sex-Linked Traits: Traits are carried on the X chromosome with differing pattern of inheritance than genes/ autosomes.
• Note. Always Never the Y chromosome*
• Who do males inherit their sex Linked traits from? Could males be carriers for same Mendalian traits? Why not?
• Sex Link and X male pattern Baldness, color and hemophilia examples. (recessive traits carried by mothers with the offspring).
• Note Females need two recessive chromosomes where males do not).*
  • Sex's must be reported on depending on answers and depending on alleles on X chromosome
  • Barr Bodies Females have two X chromosomes, of their cells with one active copy. "Shuts Off" creating codominance traits such as calicos. (Barr Body).
• Is the number affected: male , male or equal
• Is sex-linked: is not the same ratio of gender irl

PEDIGREE: RULES IN A RELATIONSHIP

• Note these are "shortcuts" if the entire families information isnt there.

1. Recessive Auto Somoal skip generation and equal on sex. Affected kids can't be created for both auto and X traits

• Note auto traits can find the traits in the parental side with one affected and the offspring do not.*

2. Recessive X linked skip generations, Males usually get it.

3. Dominant all generations, parents always get it

4. Dominant - Dominant skips generations, male > female with equal ratio.

• note there has to be more information if the results are not a definitive or do not match the given results*