5.2- Mendel and the Gene idea
blending hypothesis
idea that genetic material from the two parents blends together
Ex. blue + yellow = green
particulate hypothesis
idea that parents pass on discrete heritable units (genes)
mendel documented a particulate mechanism through his experiments with garden peas
Blending vs particulate = blend vs discrete
Mendel
discovered the basic principles of heredity by breeding garden peas in carefully planned experiments
advantages of pea plants for genetic study
many varieties with distinct heritable features, or characters (flower color); character variants (purple/ white flower) are called traits
mating of plants can be controlled
each pea plant has sperm-producing organs (stamens) and egg-producing organs (carpels)
cross-pollination (fertilization between different plants) can be achieved by dusting one plant with pollen from another
mendel chose to track only those characters that varied in an either-or manner
he also used varieties that were true-breeding
plants that produce offspring of the same variety when they self-pollinate
in a typical experiment, mendel mated two contrasting, true-breeding varieties, a process called hybridization
true-breeding parents = P generation
hybrid offspring of P generation = F1 generation
when F1 individuals self-pollinate, the F2 generation is produced
The law of segregation
when mendel crossed contrasting, true-breeding white and purple flowered pea plants, all of the F1 hybrids were purple
when mendel crossed the F1 hybrids, many of the F2 plants had purple flowers, but some had white
discovered a ratio of about three to one, purple to white flowers, in the F2 generation
PP (purple) x pp (white)
all Pp (purple phenotype)
Pp (purple) x Pp (purple)
75% purple , 25% white
why?
mendel reasoned that only the purple flower factor was affecting flower color in the F1 hybrids
mendel called the purple flower color a dominant trait and the white flower color a recessive trait
mendel observed the same pattern of inheritance in six other pea plant characters, each represented by two traits
what mendel called a “heritable factor” is what we now call a gene
mendel’s model
mendel developed a hypothesis to explain the 3:1 inheritance pattern he observed in F2 offspring
four related concepts make up this model
these concepts can be related to what we now know about genes and chromosomes
→ Mendel’s law
First concept
alternative versions of genes account for variations in inherited characters
Ex. gene for flower color in pea plants exists in two versions
one for white, one for purple
these alternative versions of a gene are now called alleles
each gene resides at a specific locus on a specific chromosome
second concept
for each character an organism inherits two alleles, one from each parent
mendel made this deduction without knowing about the role of chromosomes
two alleles at a locus on a chromosome may be identical, as in the true-breeding plants of P generation
two alleles at a locus may differ, as in F1 hybrids
third concept
if the two alleles at a locus differ
one (the dominant allele) determines the organism’s appearance
the other (the recessive allele) has no noticeable effect on appearance
in the flower-color example
F1 plants had purple flowers because the allele for that trait is dominant
fourth concept
law of segregation
states that the two alleles for a heritable character separate (segregate) during gamete formation and end up in different gametes
an egg/ sperm gets only one of the two alleles that are present in the somatic cells of an organism
this segregation of alleles corresponds to the distribution of homologous chromosomes to different gametes in meiosis
mendel’s segregation model accounts for the 3:1 ratio he observed in the F2 generation of his numerous crosses
the possible combinations of sperm and egg can be shown using a Punnett square
a diagram for predicting the results of a genetic cross between individuals of known genetic makeup
Capital letter = dominant trait
Lowercase letter = recessive trait
Useful genetic vocabulary
homozygous
an organism with two identical alleles for a character
heterozygous
an organism that has two different alleles for a gene
* Heterozygotes are NOT true-breeding
because of different effects of dominant and recessive alleles
an organism’s traits do not always reveal its genetic composition
therefore, we distinguish between an organism’s phenotype, or physical appearance, and its genotype, or genetic makeup
in the example of flower color in pea plants
PP and Pp plants have the same phenotype (purple) but different genotypes
phenotype = 3:1 (3 purple: 1 white)
genotype = 1:2:1 (1 PP 2 Pp 1 pp)
How can we tell the genotype of an individual with the dominant phenotype? PP or Pp?
such an individual must have one dominant allele
but the individual could be either homozygous dominant or heterozygous
the answer is to carry out a testcross
breeding the mystery individual with a homozygous recessive individual
if any offspring display the recessive phenotype
mystery individual = heterozygous
The law of Independent Assortment
mendel derived the law of segregation by following a single character
the F1 offspring produced in this cross were monohybrids
individuals that are heterozygous for one character
a cross between such heterozygotes = monohybrid cross
Mendel identified his second law of inheritance by following two characters at the same time
crossing two true-breeding parents differing in two characters produce dihybrids in the F1 generation, heterozygous for both characters
a dihybrid cross, a cross between F1 dihybrids, can determine whether two characters are transmitted to offspring as a package or independently
using a dihybrid cross
mendel developed the law of independent assortment
the law of independent assortment states that each pair of alleles segregates independently of each other pair of alleles during gamete formation
law applies only to genes on different, nonhomologous chromosomes
genes located near each other on the same chromosome tend to be inherited together
Difference between law of segregation vs law of independent assortment?
law of segregation
two alleles for a trait separate during gamete formation (meiosis), so each gamete carries only one allele.
one trait at a time (monohybrid)
during anaphase I of meiosis
Focus: Monohybrid cross (3:1 ratio).
law of independent assortment
genes for different traits separate independently of one another during gamete formation
Alleles of different genes separate independently during meiosis
The segregation of alleles for one gene is independent of the segregation of alleles for another gene (e.g. AaBb produces AB, Ab, aB, ab)
Focus: Dihybrid cross (9:3:3:1ratio)
Only applies to genes on different chromosomes or far apart on the same chromosome; linked genes do not follow this rule.
9:3:3:1 ratio for phenotype
9 dominant A+ dominant B
3 dominant A + recessive b
3 recessive a + dominant B
1 recessive a + recessive b
If genes are linked:
Look for:
✅ parental combinations MUCH more common
✅ recombinant types rare
Two big groups + two tiny groups = LINKED
concept 2: the laws of probability govern Mendelian inheritance
mendel’s law of segregation and independent assortment reflect the rules of probability
when tossing a coin, the outcome of one toss has no impact on the outcome of the next toss
in the same way, the alleles of one gene segregate into gametes independently of another gene’s alleles
the multiplication rule
the “and” rule
the probability that two or more independent events will occur together is the product of their individual probabilities
segregation in a heterozygous plant is like flipping a coin
each gamete has a 50% chance of carrying the dominant allele and a 50% chance of carrying the recessive allele
ex. what is the probability of both coins showing heads?
Both = “AND” multiplication
½ x ½ = ¼
this probability is the same for all genotypes in the Punnett square
the addition rule
“or” rule
the probability that any one of two or more exclusive events will occur is calculated by adding together their individual probabilities
ex. what is the probability that the offspring from a monohybrid cross will be heterozygous rather than homozygous
Rather than = “OR” addition
¼ + ¼ = ½ = 50%
solving complex genetics problems with the Rules of Probability
we can apply the multiplication and addition rules to predict the outcome of crosses involving multiple characters
in calculating the chances for various genotypes
each character is considered separately
and then the individual probabilities are multiplied together
ex. YyRr x YyRr, what is the probability of getting YYRR?
we can separate the two into their own punnett squares
Yy x Yy, Rr x Rr
= ¼ x ¼ = 1/16
concept 3: inheritance patterns are often more complex than predicted by simple Mendelian genetics
the relationship between genotype and phenotype is rarely as simplel as in the pea plant characters mendel studied
many heritable characters are not determined by only one gene with two alleles
however, the basic principles of segregation and independent assortment apply even to more complex patterns of inheritance
extending mendelian genetics for a single gene
inheritance of characters by a single gene may deviate from simple Mendelian patterns in the following situations
when alleles are not completely dominant or recessive
when a gene has more than two alleles
when a gene produces multiple phenotypes
degrees of dominance
complete dominance
occurs when phenotypes of the heterozygote and dominant homozygote are identical
incomplete dominance
phenotype of F1 hybrids is somewhere between the phenotypes of the two parental varieties
codominance
two dominant alleles affect the phenotype in separate, distinguishable ways
incomplete dominance
blend of traits (red + white → pink)
complete dominance
show both traits simultaneously (white + red → roan)
the relation between dominance and phenotype
a dominant allele does not subdue a recessive allele
alleles dont interact
alleles are simply variations in a gene’s nucleotide sequence
for any character, dominance/ recessiveness relationships of alleles depend on the level at which we examine the phenotype
Tay-Sachs disease
fatal, a dysfunctional enzyme causes an accumulation of lipids in the brain
at the organismal level, the allele is recessive
at the biochemical level, the phenotype (i.e., the enzyme activity level) is incompletely dominant
at the molecular level, the alleles are codominant
frequency of dominant alleles
dominant alleles are not necessarily more common in populations than recessive
multiple alleles
most genes exist in populations in more than two allelic forms
ex. four phenotypes of ABO blood group in humans are determined by three alleles for the enzyme (I) that attaches A or B carbohydrates to red blood cells IA, IB, and i
the enzyme encoded by the IA allele adds the A carbohydrate, IB adds B, i adds neither
pleiotropy (one gene can influence multiple phenotypic traits)
most genes have multiple phenotypic effects, a property called pleiotropy
ex. pleiotropic alleles are responsible for the multiple symptoms of certain hereditary diseases
such as cystic fibrosis and sickle-cell disease
epistasis
a gene at one locus alters the phenotypic expression of a gene at a second locus
ex. in mice and many mammals, coat color depends on two genes
one gene determines the pigment color (with alleles B for black and b for brown)
the other gene (with alleles C for color and c for no color) determines whether the pigment will be deposited in the hair
9:3:4 ratio → Recessive epistasis
Occurs when homozygous recessive of one gene masks the effect of another gene.
the ee genotype is epistatic (masks other gene).
2:3:1 ratio → Dominant epistasis
Occurs when a dominant allele of one gene masks the effect of another gene.
: A_ is dominant epistatic, masks B/b. (A_B_)
9:7 ratio → Complementary gene interaction
Occurs when two genes work together to produce a phenotype.
both dominant alleles are needed for the trait.
polygenic inheritance
quantitative characters are those that vary in the population along a continuum
quantitative variation usually indicates polygenic inheritance
an additive effect of two or more genes on a single phenotype
skin color in humans is an example of polygenic inheritance (multiple genes code for skin color)
Additive Effect: Each gene variant (allele) contributes a small, often equal amount to the phenotype, and these effects accumulate
the environmental impact on phenotype
the norm of reaction is the phenotypic range of a genotype influenced by the environment
The range of possible phenotypes that a genotype can produce under different environmental conditions is called the norm of reaction.
ex. hydrangea flowers of the same genotype range from blue-violet to pink, depending on soil acidity
integrating a Mendelian view of heredity and variation
an organism’s phenotype includes its physical appearance, internal anatomy, physiology, and behavior
an organism’s phenotype reflects its overall genotype and unique environmental history
concept 4: many human traits follow Mendelian patterns of inheritance
humans are not good subjects for genetic research
generation time = too long
parents produce relatively few offspring
breeding experiments are unacceptable
however, basic Mendelian genetics endures as the foundation of human genetics
pedigree analysis
pedigree = a family tree that describes the interrelationships of parents and children across generations
inheritance patterns of particular traits can be traced and described using pedigrees
recessively inherited disorders show up only in individuals homozygous for the allele
carriers are heterozygous individuals who carry the recessive allele but are phenotypically normal (i.e. pigmented)
albinism is a recessive condition characterized by a lack of pigmentation in skin and hair
if a recessive allele that causes a disease is rare, then the chance of two carriers meeting and mating is low
consanguineous mating (matings between close relatives) increase the chance of mating between two carriers of the same rare allele
most societies and cultures have laws of taboos against marriages between close relatives
dominantly inherited disorders
some human disorders are caused by dominant alleles
dominant alleles that cause a lethal disease are rare and arise by mutation
Achondroplasia = a form of dwarfism caused by a rare dominant allele
How to identify pedigrees
#1 Recessive
unshaded parents have a shaded child
Logic: if neither parents shows the trait, but they pass it on, the trait must be hidden. only recessive traits can hide
Conclusion: the trait is recessive. both parents are heterozygous (carriers)
#2 Dominant
look for two parents who are shaded but have an unshaded child
logic: if both parents show the trait, but the child is normal, the normal gene is hidden. if the trait were recessive, two affected parents could only have affected children
conclusion: the trait is dominant. both parents are heterozygous
#3 X-linked
affects significantly more males than females
logic: males (XY) only have one X chromosome. if they inherit one bad gene, they have the trait. females need two (XX)
evidence: an affected mother must have all affected sons. if an unaffected father has an affected daughter, the trait cannot be X-linked recessive
#4 Y-linked clue
only appears in males and never skips a male in a lineage
logic: only makes have a Y chromosome, they pass it directly to their sons
conclusion: if ONLY males have the trait, and every affected father has 100% affected sons (and no affected daughter), it is Y-linked
autosomal dominant
appears in every generation, does not skip
males and females affected equally
if one parent heterozygous→ about 50% children affected
affected person usually has an affected parent
autosomal recessive
can skip generations
unaffected parents have affected child
males and females equally affected
may appear more with consanguinity
X-linked recessive
more affected males than females
NO father to son transmission
affected father → daughters are carriers
can skip generation through female carriers
affected sons often have carrier mothers
X linked dominant
appears every generation
affected father = all daughters affected
affected father → no sons affected
females more than males
keys
check male vs female patterns
mostly males affected → x linked recessive
no father to son transmission
both sexes equally affected → likely autosomal
does it skip generation
yes → recessive
no → dominant
look at parents → child patterns
dominant
affected child = unaffected parent
recessive
unaffected parents = affected child
x linked recessive
mostly males
affected father → ALL daughters carriers
affected father → no affected sons
affected mother → ALL sons affected
x linked dominant
affected father → all daughters affected
affected father → no sons affected
huntington’s disease
a degenerative disease of the nervous system
has no obvious phenotypic effects until the individual is about 35-40 years of age
fetal testing
in amniocentesis (羊膜穿刺術), the liquid that bathes the fetus is removed and tested
in chorionic villus sampling (CVS), a sample of the placenta is removed and tested
other techniques, such as ultrasound and fetoscopy, allow fetal health to be assessed visually in utero
