topic 1: mendelian inheritance Flashcards
children do not inherit particular physical traits, but rather
genes
what is heredity?
the transmission of traits from one generation to the next
what is variation?
the differences in appearance between offsprings and their parents
what are the 2 laws of inheritance discovered by Gregor Mendel?
- law of segregation
- law of independent assortment
what are some advantages of using pea plants for genetic study?
- many different characteristics and traits
- mating of plants can be controlled
- each pea plant has sperm and egg producing organs
- cross pollination
what are sperm-producing organs and egg-producing organs known as?
stamens - (sperm-producing)
carpels - (egg-producing)
what is the difference between characteristics and traits?
characteristics - distinct heritable features (ex: flower color)
traits - characteristic variants (ex: purple or white flower)
what is cross-pollination?
fertilization between different plants
(can be achieved by dusting one plant with pollen from another)
what are true-breeding varieties?
- homozygotes
- plants that produce offspring of the same variants only when they self-pollinate
what is hybridization?
mating 2 CONTRASTING true-breeding plant varieties
(ex: white and purple flowered pea plants)
if all F1 plants had purple flowers after mating true-breeding white and purple flowered plants, what conclusion does that give?
purple flowers are dominant, while white flowers are recessive
if all F1 plants had purple flowers after mating true-breeding white and purple flowered plants, what is the ratio of purple to white flowers in the F2 generation?
3:1
what is meant by homozygous?
an organism with 2 identical alleles for a character for a gene controlling that character
(ex: AA or aa; true-breeding)
what is meant by heterozygous?
an organism with 2 different alleles for a character for a gene controlling that character
(ex: Aa; hybrid)
what is the difference between phenotype and genotype?
phenotype - the physical appearance of an organism
genotype - the genetic make up of an organism
what are dominant and recessive alleles?
dominant alleles - the ones expressed in the phenotype
recessive alleles - not expressed in the phenotype when combined with a dominant allele
what are the 4 related concepts explaining the 3:1 inheritance pattern Mendel observed in F2 offspring?
1 - alternative versions of genes (alleles) account for variation in inherited characteristics
2 - an organism inherits 2 alleles for each characteristic
3 - if the 2 alleles differ at a locus, then the dominant allele determines the phenotype of an organism while the recessive allele has no noticeable feature
4 - the law of segregation
what does the law of segregation state?
segregation of alleles corresponds to the separation of homologous chromosomes to different gametes during meiosis
what is a Punnett square used for?
a diagram that predicts the results of a genetic cross between individuals of known genetic makeup
how can we tell the genotype of an individual with the dominant phenotype?
by carrying out a testcross
how is a testcross carried?
by breeding the unknown genotype individual with a homozygous recessive individual
if the offspring displays the recessive phenotype in a testcross, then the unknown parent must be?
heterozygous
what are monohybrids?
organisms that are heterozygous for ONE character
Mendel derived the law of segregation through the inheritance pattern of?
monohybrids
Mendel derived the law of independent assortment using a?
dihybrid cross
what is a dihybrid cross?
crossing 2 true-breeding parents differing in 2 characteristics, produces dihybrids in F1 heterozygous for both charateristics
what is the one requirement for the law of independent assortment to be applied?
2 genes must be located on DIFFERENT chromosomes
(non-homologous chromosomes)
what does a dihybrid cross determine?
whether 2 characteristics are transmitted to offspring as a package (together) or separately
what does the law of independent assortment state?
each pair of alleles segregates independently of one another during gamete formation
what is the difference between the phenotypic ratio produced by an independent assortment and a dependent assortment?
law of independent assortment - produces a 9:3:3:1 ratio (genes are located on different chromosomes)
law of dependent assortment - produces a 3:1 ratio (genes are located on the same chromosomes or are LINKED)
when may inheritance deviate from simple Mendelian patterns?
- when alleles are not completely dominant or recessive
- when a gene has more than 2 alleles
- when a gene produces multiple phenotypes
what are the different types of dominance?
- complete dominance
- incomplete dominance
- codominance
what is complete dominance?
when phenotypes of the heterozygote and dominant homozygote are identical
(expression of dominant allele phenotype)
what is incomplete dominance?
when the phenotype of heterozygotes (F1 hybrids) is somewhere between the phenotypes of the 2 parental phenotypes
(neither allele is expressed; intermediate phenotype)
what is codominance?
2 dominant alleles affect the phenotype in separate distinguishable ways
(both alleles are expressed)
what are examples of incomplete dominance?
- palomino horses
- pink flowers (from crossing of white and red flowers)
what is an example of codominance?
human blood group types
describe the 3 alleles in human blood group types
A and B = codominant alleles
O = recessive allele
AA or AO - blood group A
BB or BO - blood group B
AB - blood group AB
OO - blood group O
what is an example of multiple allelism?
human blood group types
what do the 4 phenotypes of the ABO blood group (A, B, AB, O) depend on?
- RBC membrane carbohydrates
- 3 alleles for the enzyme (I) that attaches A or B carbohydrates to RBCs: IA, IB, i
what does each allele of the (I) enzyme add?
IA allele - codes for enzyme that adds A carbohydrate
IB allele - codes for enzyme that adds B carbohydrate
i allele - codes for enzyme that adds neither
what is an example of a recessive allele being more prevalent than a dominant allele
having 6 fingers is caused by a dominant allele, which is way less common in a population than having 5 fingers, caused by a recessive allele
what is pleiotropy?
a property of most genes to have multiple phenotypic effects
what are examples of hereditary diseases that have multiple symptoms caused by pleiotropic alleles?
- sickle-cell disease
- cystic fibrosis
what are some ways traits can be determined/controlled by 2 or more genes?
- epistasis: when the effects of 1 gene are modified by other genes
- polygenic inheritance: when 2 or more genes control the expression of a single phenotype
what is epistasis? give an example
- a gene at one locus alters the phenotypic expression of a gene at a 2nd locus and results in 2 different gene sets affecting the same phenotypic characteristic
- example: coat color of mice depends on 2 genes
- one gene determines pigment color
(B for black, b for brown) - the other gene determines where the pigment will be deposited in the hair
(C for color, c for no color)
what is an example of a human epistatic disease?
albinism
describe albinism
a congenital autosomal recessive disorder characterized by the complete or partial absence of pigment in the skin, hair, eyes
how is albinism caused?
it is caused due to the absence of tyrosinase, the enzyme responsible for the production of melanin
what is polygenic inheritance? what is it characterized by?
- when 2 or more genes control the expression of a single phenotype
- usually characterized by quantitative variation
what are quantitative characteristics? what are examples of polygenic inheritance?
- characters that vary in the population along a continuum (continuous variation)
- height and skin color in humans are examples of polygenic inheritance
what are some environmental conditions (internal/external) that affect some phenotypes?
- temperature
- chemicals
- nutritional habits
what are multifactorial characteristics?
characteristics where both genetic and environmental factors influence phenotype
what is meant by norm of reaction? give an example
the phenotypic range of a genotype influenced by the environment
(broadest for polygenic characteristics)
ex: hydrangea flowers of the same type range from blue-violet to pink, depending on soil acidity
what does an organism’s phenotype include?
- its physical appearance
- internal anatomy
- physiology
- behavior
why would humans not be good subjects for genetic research?
- generation time is too long
- parents produce relatively few offspring
- breeding experiments are unacceptable
- unethical
what are the common features of homologous chromosomes?
- have the same length and shape
- carry genes controlling the same inherited characteristics
- do not necessarily carry the same alleles
what are pedigrees? what are they used for?
- a family tree that describes the interrelationships of parents and children across generations
- used to make predictions about future offspring and finding the probability of getting children
how can you determine if a trait is dominant or recessive from a pedigree?
dominant - if 2 affected parents have an unaffected child
recessive - if 2 unaffected parents have an affected child
what are two examples where pedigree diagrams were used to determine cause of conditions?
- endogamy in kingdoms
- Habsburg lip
inherited human disorders can be of 3 types, which are?
- autosomal dominant
- autosomal recessive
- sex-linked
what are examples of autosomal recessive diseases?
- cystic fibrosis
- albinism
- sickle cell anemia
- α/β thalassemia
describe autosomal recessive disorder
- due to a mutation in an allele in one of the 22 autosomes
- the mutant allele is recessive
- AA or Aa = healthy individuals
- aa = individuals with disease
recessively inherited disorders show up only in what individuals?
in individuals homozygous for the allele
what are carriers?
heterozygous individuals who carry the recessive allele but are phenotypically normal
why are autosomal recessive disorders more common than autosomal dominant disorders?
- because the disease alleles “hide” in the healthy heterozygous carriers that have a normal phenotype
- in autosomal dominant, the disorder is always expressed phenotypically
what are consanguineous matings?
- mating between close relatives
(taboo or illegal in most societies/cultures) - increase the chance of mating between 2 carriers of the same rare allele
what causes sickle-cell anemia?
the substitution of a single amino acid (Glu -> Val) in the hemoglobin protein in RBCs
what are the symptoms of sickle-cell anemia? how are heterozygotes affected?
- physical weakness
- organ damage
- pain
- paralysis
- heterozygotes are usually healthy but may suffer some symptoms
- less susceptible to malaria parasite
describe how sickle-cell anemia arises
- substitution mutation of glutamic acid to valine (glu –> val) leads to exposed hydrophobic region
- hemoglobin molecules interact with one another, and crystallize into a fiber, greatly reducing the capacity to carry oxygen (meant to be separated so they can each carry oxygen)
- fibers of abnormal hemoglobin deform red blood cell into sickle shape
what causes thalassemia?
a mutation in the hemoglobin gene (mutations in the α or β chains cause α or β thalassemia respectively)
describe cystic fibrosis
- the cystic fibrosis allele results in defective or absent chloride transport channels in plasma membranes
- it is characterized by scarring (fibrosis) and cyst formation within the pancreas
- symptoms include: mucus buildup in some internal organs and abnormal absorption of nutrients in small intestine
what are examples of autosomal dominant diseases?
- Huntington disease
- Achondroplasia
- Marfan syndrome
- Polycystic kidney disease
what are examples of X-linked diseases?
what is the most common type of genetic disorders?
autosomal recessive disorders
