genetics Flashcards
what is the blending theory of inheritance?
black bunny + white bunny = gray bunny
- not true, too much variation
what is the particulate theory of inheritance?
parents pass on discrete, heritable traits “particles”
why are garden peas a good model organism to study genetics?
- easy-to-see traits (shapes, colors, sizes)
- controlled matings
- many offspring
- short generation time
what is an allele?
hereditary factors resulting in contrasting traits/ variance in a gene
what is a genotype?
the combination of alleles in an individual
what is a phenotype?
the expression of a trait (physical characteristic) ex: 3:1 ratio dominant:recessive
what is segregation(by mendel)
genes come in pairs that separate from one another in formation of genes, during anaphase I of meiosis, separation of homologous chromosomes
Pp
heterozygous
PP or pp
homozygous (2 of same allele)
Pp PP pp
all genotypes. phenotype: color from presence/absence of an enzyme
what is independent assortment?
alleles for different genes segregate independently of one another during gamete formation. This means that the inheritance of one trait does not affect the inheritance of another (as long as the genes are on different chromosomes or far apart on the same chromosome).
- occurs in metaphase I ( each pair segregates differently)
what is character?
a heritable feature that varies among individuals (flower color)
what is a trait?
each variant for a character (purple or white flower)
what is true breeding?
organisms that produce offspring of some variety over many generations of self-pollination
what is hybridization?
mating/crossing of 2 true-breeding varieties
what is mendel’s model?
- alternative versions of genes account for variations in inherited characters
- for each character, an organism inherits 2 versions (alleles) of a gene, one from each parent
- if the 2 alleles at a locus differ, then the dominant allele determines organism’s appearance
- law of segregation (the 2 alleles for a heritable character separate from each other during gamete formation and end up in different gametes
what is testcross?
breeding an organism of unknown genotype with a recessive homozygote can reveal genotype of organism
what are monohybrids?
heterozygous for a character being followed in testcross - monohybrid cross
what are dihybrids
individuals heterozygous for the 2 characters being followed in cross - dihybrid cross
what is the law of independent assortment?
two or more genes assort independently during gamete formation (metaphase I)
what is the multiplication rule?
to determine the probability of one event and the other occurring, we multiply probability of each event (1/2 x 1/2 = 1/4)
what is the addition rule?
the probability that one event or the other (mutually-exclusive) will occur is calculated by adding each probability (1/4 + 1/4 = 1/2)
Autosomal Recessive vs. Autosomal Dominant Inheritance
autosomal recessive: two mutant alleles (aa), heterozygous individuals are carriers, both parents must have to pass to child, often skips generations, cystic fibrosis and sickle cell anemia and tay-sachs
autosomal dominant: one mutant allele (A_), no carriers bc individual with one mutant allele expresses the trait, affected parent has 50% chance of passing, affects each generation, huntingtons (diagnosed late = fatal) and marfan and achondroplasia (dwarfism)
what can one copy of sickle-cell reduce
malaria
what tests can identify carriers?
fetal testing (screening and diagnostic testing), imaging and blood test screening, diagnostic (amniocentesis detects serious recessive disease, chorionic villus sampling), newborn screening
what is autosomal dominant
only one mutated allele needed. there are lots of genes on X chromosome. Y chromosome = “genetic wasteland”, much smaller than X. males are hemizygous for sex-linked genes (an individual who has only one copy of a gene, rather than the usual two)
How are Deleterious (harmful) Alleles Are Maintained in the Gene Pool
- Recessive deleterious alleles (aa) can persist because carriers (Aa) do not express the harmful trait.
- Late-Onset Disorders
Example: Huntington’s disease symptoms develop after 30-40 years, allowing affected individuals to pass on the allele before symptoms appear. - Mutation and New Introductions
Harmful alleles can continuously arise due to random mutations.
Even if selection removes some, mutations reintroduce them into the gene pool. - Balancing Selection
When natural selection maintains genetic variation in a population because different alleles provide advantages in different conditions.
Example: The G6PD deficiency allele is common in malaria-endemic regions because it reduces malaria severity.
why are large samples needed for genetic research?
- reduces random error, increases accuracy
- captures genetic variatoin
- accounts for outside influences
Explain the difference between the number of alleles for a given gene that are found in an individual vs
the number of alleles for a given gene that can be found in a population
In an Individual:
- Each person has two alleles for a given gene (one from each parent).
- Example: For the ABO blood group, an individual can have only two alleles (e.g., A and O).
In a Population:
- A population can have multiple alleles for the same gene, beyond just the two found in any individual.
- Example: The ABO blood group has three alleles (A, B, and O) present in the population, though an individual only inherits two.
Explain who to distinguish between complete dominance, codominance, and incomplete dominance
Complete Dominance
- One allele completely masks the effect of the other.
- Heterozygotes (Aa) express only the dominant trait (A).
- Example: Mendelian pea plants – Purple (A) is dominant over white (a), so Aa = purple flowers.
Codominance
- Both alleles are fully expressed in heterozygotes.
- No blending—both traits appear simultaneously.
- Example: AB blood type – A and B alleles are both equally expressed, resulting in AB blood.
Incomplete Dominance
- Blending of traits occurs in heterozygotes.
- The heterozygous phenotype is intermediate between the two homozygous phenotypes.
- Example: Red (RR) × White (WW) snapdragons → Pink (RW) offspring.
Understand the inheritance and molecular basis of the human ABO blood group
- Inheritance Pattern
- The ABO blood group follows codominant inheritance.
- The ABO gene (on chromosome 9) has three alleles:
Iᴬ (produces A antigen)
Iᴮ (produces B antigen)
i (produces no antigen, O type)
- Iᴬ and Iᴮ are codominant, meaning both are expressed if inherited together (AB blood type).
- i is recessive, so individuals with ii have blood type O.
Genotype Phenotype (Blood Type)
IᴬIᴬ or Iᴬi A
IᴮIᴮ or Iᴮi B
IᴬIᴮ AB
ii O - Molecular Basis
- The ABO gene encodes an enzyme (glycosyltransferase) that modifies sugar molecules on red blood cell surfaces.
- Iᴬ allele → Adds N-acetylgalactosamine (A antigen).
- Iᴮ allele → Adds galactose (B antigen).
- i allele → Non-functional enzyme, no sugar modification (O blood type has no antigens). - Clinical Relevance
- ABO blood type is crucial for blood transfusions (incompatible transfusions cause immune reactions).
- Universal Donor: Type O (no antigens).
- Universal Recipient: Type AB (has both A & B antigens, so no antibodies against either).
- This system demonstrates Mendelian inheritance, codominance, and biochemical differences at the molecular level.
Describe an allelic series and understand that an allele can be dominant to one allele and recessive to another
- What is an Allelic Series?
An allelic series refers to multiple alleles of a single gene that have a hierarchy of dominance relationships. Some alleles may be completely dominant, recessive, or show partial dominance over others. - Example of an Allelic Series
One classic example is the C gene in rabbit coat color, which has multiple alleles with a dominance hierarchy:
C (full color) > cʰ (Himalayan) > cʙ (chinchilla) > c (albino)
C is dominant to all other alleles.
cʰ is dominant to cʙ and c, but recessive to C.
c is fully recessive to all other alleles.
3. An Allele Can Be Both Dominant and Recessive
A single allele can be dominant to one allele but recessive to another, depending on the combination.
Example: In the rabbit coat color allelic series:
cʰ (Himalayan) is dominant over c (albino) → cʰc = Himalayan
cʰ (Himalayan) is recessive to C (full color) → Ccʰ = full color
This concept helps explain how multiple traits arise from a single gene, leading to variation within a species.
Distinguish between pleiotropy and polygenic traits at a basic level
Pleiotropy: One gene, many traits
Polygenic inheritance: Many genes, one trait
Contrast the inheritance of sex-linked traits and autosomal traits
- Sex-linked traits depend on sex chromosomes (X or Y), while autosomal traits are on non-sex chromosomes.
- Males are more affected by X-linked recessive traits since they only have one X chromosome.
- Autosomal traits affect both sexes equally.
Explain the importance of the Y chromosome in males
- Sex Determination (SRY Gene)
The Y chromosome contains the SRY gene (Sex-determining Region Y), which is responsible for initiating the development of male characteristics.
The SRY gene triggers the development of testes from the undifferentiated gonads, which leads to the production of male hormones (androgens) like testosterone.
Testosterone further drives the development of male sexual characteristics, such as male reproductive organs and secondary sexual traits (e.g., body hair, deep voice). - Spermatogenesis
The Y chromosome contains genes crucial for sperm production. These genes are involved in the formation and function of the testes, which are essential for the production of sperm (the male gametes).
Mutations or deletions of key genes on the Y chromosome can lead to infertility. - Inheritance
The Y chromosome is passed from father to son and is inherited in a paternal lineage. This allows it to be used in tracing male ancestry through generations. - Dosage Compensation
Since males only have one X chromosome and one Y chromosome, they lack a second X chromosome to compensate for potential genetic imbalances. This makes the X chromosome in males highly significant for gene expression and functioning, while the Y chromosome mainly influences male sex characteristics.
how is eukaryotic gene expression regulated?
regulation of chromatin structure
- histone modifications and DNA methylation
- epigenetic inheritance (methyl-group containing compounds can change traits, mutations to DNA are permanent but modifications to chromatin are reversible)
how is transcription initiation regulated
control elements (segments of noncoding DNA that serve as binding sites for the proteins called transcription factors)
