Chapter 14 - Mendel and Gene Idea

Question 1

Character

Answer 1

an observable heritable feature that can vary among individuals

Question 2

Trait

Answer 2

one or two or more detectable variants in a genetic character

Question 3

True Breeding

Answer 3

organisms which produce offspring of the same variety over many generations of self-pollination
- organism will always have a homozygous genotype

• The pea plant will only have 1 allele e.g. if you keep planting the purple flower, you only get the purple flower in offspring

Question 4

Hybridization

Answer 4

the mating or crossing of two true breeding varieties

Question 5

Generation Terms

Answer 5

P Generation: the true breeding parents which produce F1 hybrid offspring

F1 Generation: the first filial heterozygous generation from a P cross

F2 Generation: the second filial generation from interbreeding between F1 generation

Question 6

Advantages leading to Mendel using Peas

Answer 6

• different alleles with distinct heritable features
  • Mating of plants could be controlled
  • Had stamens(pollen/sperm) carpels(eggs)
    • Able for Cross pollination

Question 7

Two Theories of Inheritance before Mendel:

Answer 7

Blending hypothesis: characters in the parents are mixed and offspring get a mixture of both
- The original phenotypes of the parents could be lost

Particulate Hypothesis: particles that the parents have are passed on to the offspring and depending on which particle the offspring gets is going to be shown

Question 8

Why mendel didnt agree with blending theory

Answer 8

because of the fact that the heritable factor for white flowers was only present in every 4 flowers reasonaed that the white factor didn’t disappear but was masked by the purple color
- Leading to the idea of dominant and recessive traits

Question 9

Components of Mendels theory to explain 3:1 inhertance pattern

Answer 9

1. Alternate versions of genes account for variations in inherited characters
   E.g. the flower color gene has two possibilities due to different coded nucleotides in a sequence

   2. For each character, an organism inherits two versions, one from each parent
   
    3. If two alleles at a locus differ then the dominant allele determines the organisms appearance while the recessive allele has no effect on organism appearance
   
      4. Law of Segregation

Question 10

Law of Segregation

Answer 10

Two alleles for a heritable character segregate during gamete formation and end up in different gametes

- An allele from sperm and egg will be used to create a gamete, not two alleles from sperm and none from egg

- If both alleles are different then 50% of the gametes get the dominant and 50% get the recessive allele

Question 11

What do Punnet Squares Predict

Answer 11

allele composition of offspring from a cross between individuals of a known genetic makeup
- Capital letters symbolize Dominant, lower case symbolizes recessive

Question 12

Test Cross

Answer 12

• Used to determine the genotypes of organisms who show the dominant allele type but could either be Pp or PP
  • To do so, we cross the unknown organism with a homozygous recessive organism
    
    • This means the allele in the unknown organism determines the appearance of the offspring allowing us to determine the genotype

Question 13

Second Law of Inheritance

Answer 13

1. Law of Independent Assortment: Alleles for one gene segregate into gametes independently of the alleles of another gene
   
   • Only applies to genes located on different chromosomes(non-homologs) or genes far apart on the same chromosome
     
     • found by following 2 characters through inheritance
   • Leads to phenotypical ratio is 9:3:3:1

Question 14

Alternative Theory of Law of Independent Assortment

Answer 14

2. Law of Dependent Assortment: every time it is going to be one allele, it simultaneously means a specific type of the 2nd allele is likely to follow
   
   • e.g. every time a pea is green, it is likely to be round
   • Results in a 3:1 phenotypical ratio with only the phenotypes of the P generation
   • results did not align with this hypothesis

Question 15

Dihybrids and what they prove

Answer 15

individuals heterozygous for the two characters being followed in the cross.
- e.g. pea color and pea texture

• Answers the question, by doing a dihybrid cross, that the alleles for each gene segregate independently of those of the other

Question 16

How to SOlve Complex Mendelian Genetic Crosses

Answer 16

1. Start by listing all the genotypes
2. Calculate the probability for each of these genotypes by multiplying together the individual probabilities for allele pairs from cross of PpYyRr x Ppyyrr
3. Use addition rule to add the probabilities for all different genotypes

Question 17

What are mendelian genetics good for explaining/its limitations

Answer 17

inheritance when:
- each character is determined by one gene,
- there are only two alleles
- there is a dominant and recessive allele

Question 18

When are mendelian genetics not good for explaining inheritance

Answer 18

• Inheritance of characters determined by a single gene deviate from simple mendelian patterns when:
  1. alleles are not completely dominant or recessive,
  2. when a genes has more than two alleles

           3. when a single gene produces multiple phenotypes

Question 19

Degrees of Domianance

Answer 19

Alleles can show different degrees of dominance and recessiveness in relation to each other all in heterozygotes

• complete
• incomplete
• codominance

Question 20

Complete Dominance

Answer 20

a form of mendelian genetics in which the phenotypes of the heterozygote and dominant homozygote are indistinguishable
- When the dominant masks the recessive phenotype and looks like one of the parents
- 3:1 ratio

Question 21

Incomplete Dominance

Answer 21

situation in which the phenotype of heterozygotes results in a nonparental phenotype due to a mix between the phenotypes of individuals heterozygous for either allele
- E.g. the F1 offspring of flowers being colored pink when a red flower is crossed with a white flower
- Phenotypical ratio is same as genotypic ratio(1:2:1)
- Differs to results of mendelian genetics as an organism containing both alleles(Pp) would show the phenotype of dominant color.
- Doesn’t prove blending hypothesis because alleles for color are heritable factors which maintain their identity and parental phenotypes can be found in subsequent generations

Question 22

Codominance

Answer 22

a situation in which the phenotypes of both alleles are equally dominant and exhibited in the heterozygote because they affect in distinguishable and separate ways.

- Non mendelian genetics
    - Phenotypic Ratio is the same but Genotypic ratio is 1:2:1

Question 23

Relationship between Dominance and Phenotype - how is a dominant allele expressed over a recessive allele?

Answer 23

• A gene is dominant because its seen in the phenotype and not because it subdues a recessive allele
  
  • D and R genes do not interact but the pathway from genotype to phenotype is where D and R comes into play
  • 1 dominant allele is enough to result in a certain enzyme being synthesized to carry out functions to display that phenotype opposed to the phenotype represented by the recessive phenotype.
• depends at the level the phenotype is examined
• organismal level
• biochemical level
• molecular level

Question 24

E.g. of Phenotypic differences at Different Levels

Answer 24

TaySach disease is a disease where the brain cells of infected children cannot metabolize certain lipids because a enzyme does not function properly resulting in the accumulation of lipids which causes issues.
- As only children with two copies of the TaySachs Allele has the disease, at the organismal level it is considered recessive
- However because the activity of the lipid metabolizing enzyme is intermediate, at the biochemical level it is characteristic of incomplete dominance because half the normal enzyme function is sufficient to prevent lipid accumulation
- However at the molecular level the two alleles(normal and tay sach allele) appears to be codominant because heterozygous individuals can produce equal numbers of normal and dysfunctional enzyme molecules

Question 25

Frequency of Dominant Alleles

Answer 25

• Not always the case the dominant allele is more common than the recessive allele
  ○ The allele for 5 digits per appendage is recessive but far more prevalent than the dominant allele for polydactyly(extra digits)
• lethal dominant alleles are extremely rare because they often lead to death of the organism before reproduction and passing on off the genes

Question 26

E.g.of Genes with Multiple alleles

Answer 26

• genes which have more than 2 alleles
• E.g. Blood type which is has an example of codominance

Question 27

Epistasis

Answer 27

the phenotypic expression of a gene at one locus which alters a completely different gene at a second locus
- These genes still follow the law of independent assortment when breeding
- Phenotypical ratio of 9:3:4

Question 28

Example of Epistasis

Answer 28

in labradors, black color coat is dominant to brown however there is a second gene determine whether pigment will be deposited in hair
- This gene has a dominant allele which deposits either the black or brown pigment but its recessive gene makes the coat yellow regardless of black/brown gene
So pigment deposition is epistatic(alters the gene at a second locus) to the gene which codes for black or brown

Question 29

Polygenic Inheritance

Answer 29

Quantitative characters: cannot be classified on an either or basis and instead vary along a continuum due to having a phenotype composed of more than 2 genes

• no dominant or recessive genes, instead alleles have an additive effect

E.g. height and skin color

Question 30

Pleiotrophy

Answer 30

the ability of a single gene to have multiple phenotypic effects
- E.g. the gene determining flower color can also affect the color of coating on surface of seeds
- Includes diseases such as sickle-cell disease which is a recessive disease and people heterozygous for these diseases are protected from malaria due to the mutation in hemoglobin

Question 31

Multifactorial Inheritance

Answer 31

phenotypically characters which are influenced by multiple genes and environmental factors
- E.g. hydrangia in which the color of the flow is affected by the soil pH

Question 32

Recessively Inherited Disorders:

Answer 32

• a disease requiring two recessive alleles to be shown in the phenotype
• most people infected are born to carriers
• Heterozygotes don’t have the phenotype but are carriers because they can transmit the recessive allele to their offspring
• same family meeting increases the likelihood of recessive disorders due to them more likely to pass along the same recessive trait

Question 33

Cystic Fibrosis

Answer 33

a human genetic disease caused by a recessive allele and is characterized by an excessive secretion of mucus and consequent vulnerability to infection.
- The normal allele codes for a gene that functions in the transport of chloride ions
○ The result of two recessive genes is an abnormally high concentration of intracellular chloride, causing an uptake of water and creating greater amounts of mucus to build up in pancreas, lungs, digestive tract and other organs

Question 34

Sickle Cell Disease

Answer 34

a recessive condition caused by the substitution of a single amino acid in the hemoglobin protein of red blood cells leading to sickled cells clumping and clogging small blood vessels
- The presence of 1 affected allele can affect the phenotype by providing malaria resistance and so at the organismal level the normal allele is incompletely dominant.
- At the molecular level, the two alleles are codominant and are made in heterozygotes meaning that in the long term the heterozygote can experience symptoms

Question 35

Example of Dominantly Inherited Disorders: Achondrofplasia

Answer 35

a form of dwarfism in which heterozygous individuals have the dwarf phenotype and 99.99% of the population are homozygous recessive

Question 36

Example of Dominantly Inherited Disorders: Huntington’s Disease

Answer 36

caused by a lethal dominant allele that has no obvious phenotypic effect until the individual is about 35-45 years old
- When it does it includes the deterioration of the NS which is irreversible and fatal

Question 37

Multifactorial Diseases

Answer 37

genetic diseases which have a significant environmental influence
E.g. heart disease, diabetes, cancer, alcoholism and certain mental illnesses.