Offspring traits were a 50/50 mixture of the traits of each parent.
Inheritance of Acquired Traits (Lamarck)
Traits acquired during an individual's lifetime could be passed on to their offspring. Use or disuse of a structure would cause the structure to become more or less developed, respectively.
But does it really work that way?
Classical Mendelian Genetics: Single Gene Inheritance
The study of intergenerational inheritance patterns
is also known as transmission genetics.
Gregor Mendel published Versuche uber Pflanzenhybriden
(Experiments on Plant Hybridization) in 1866.
Although its importance was not recognized at its publication,
Mendel's work eventually changed the course of biology.
Quantitative Traits
The earliest studies of inheritance focused on quantitative traits.
A quantitative (= continuous) trait
is a phenotype resulting from the cumulative actions of more than one gene.
is thus said to be polygenic.
can vary among individuals to produce a continuous distribution of phenotypes.
examples:
stature/height
eye color
hair/fur color
cranium size
body scent
color pattern of shells
flower shapes and colors
actually....most traits
Add the effect of environmental influence on the traits,
and you get something very difficult to characterize.
Because of the complex inheritance pattern of quantitative traits,
there was little true understanding of inheritance.
And then along came Gregor Mendel.
Qualitative Traits
Mendel was the first to investigate qualitative traits.
A qualitative (= discrete) trait
is a phenotype resulting from the action of a single gene.
is thus said to be monogenic.
expressed as a discrete phenotype.
examples:
human ABO blood types
human ear wax - wet vs. dry
human cystic fibrosis
human Huntington's disease
human Sickle Cell Anemia
dwarfism in many mammals
many traits in sweet peas
actually....most traits
Because of their relatively simple "either/or" inheritance patterns,
qualitative traits could more easily be characterized than quantitative traits.
As Mendel discovered, it's a simple matter of statistics.
Mendel's Laws of Genetics
The simple inheritance patterns of qualitative traits allowed Mendel to devise two Laws of Genetics:
Law of Segregation (Mendel's First Law)
Every (diploid) individual carries two copies of every gene,
one from each parent.
When that individual produces gametes, the two gene copies
separate from each other and migrate to separate daughter cells.
Each gamete contains only one copy of the gene.
Law of Independent Assortment (Mendel's Second Law)
When an individual produces gametes,
the various genes inherited from each parent (i.e., in each set)
migrate into new daugher cells independently of one another.
Multiple Alleles --> Phenotypic Variation
A monogenic trait varies only when there is more than one allele in the population.
With respect to a monogenic trait...
wild type - the phenotype most common in a natural population
mutant - any phenotype other than wild type
Mutants for study can be obtained via
screening - examining many individuals to find mutant variants
induction - subjecting model organisms to mutagens (e.g., chemicals, ionizing radiation) to induce
heritable mutations in the germline.
Punnett Square: A Null Hypothesis
The probability of any given genotype being inherited by an offspring
from its parents can be calculated with a Punnett Square.
Since the alleles of a gene separate from each other during gamete formation,
each allele can be placed by itself on a matrix.
Complete the matrix to see the possible allele combinations and their probability.
This can be done for any number and combination of monogenic traits.
But it gets a bit unwieldy with more than two or three.
The top illustration shows a monohybrid cross.
Each parent is heterozygous (hybrid) for one trait.
genotype probabilities:
BB = 25%
Bb = 50%
bb = 25%
that is, 1:2:1
phenotype probabilities:
pink (dominant) = 75%
white (recessive) = 25%
that is, 3:1
The bottom illustration shows a dihybrid cross.
Each parent is heterozygous (hybrid) for two traits.
genotype probabilities:
1/16 (6.25%) of any combination
phenotype probabilities:
both traits dominant = 9/16
pea color dominant/pod color recessive = 3/16
pod color dominant/pea color recessive = 3/16
both traits recessive = 1/16
that is, 9:3:3:3:1
Meiosis: The Chromosomal Basis of Mendel's Laws
Understanding how chromosomes migrate during mitosis and meiosis allows us to better understand Mendel's Laws.
A diploid organism inherits only one allele of every gene from each parent.
When that individual produces gametes, the genes and alleles it inherited from its parents will migrate in specific ways.
diploid germline cells undergo meiosis to produce
haploid gametes (ova or sperm)
Meiosis: Segregation
Mendel did not know about chromosomes or meiosis.
But we now know that the migration of chromosomes
carrying their various genes (and alleles thereof)
is the explanation for Mendel's Law of Segregation.
Meiosis: Independent Assortment
When an individual undergoes meiosis, its
maternal and paternal chromosomes segregate at meoisis I.
They do so independently of one another.
This means that the alleles of maternal and paternal chromosomes
can end up in different, random combinations.
Transmission Genetics Terminology
Mendelian genetics is also known as transmission genetics.
Understanding its terminology is the first step to understanding its mechanisms.
autosome
a trait encoded by gene(s) on one or more autosomes
sex chromosome
a chromosome involved in determining the sex of an organism.
examples: X and Y chromosome of mammals; Z and W chromosomes of birds.
sex-linked trait
a trait encoded by gene(s) on a sex chromosome
example:
a gene located on the X chromosome
is X-linked
a gene located on the Y chromosome is Y-linked
cross
a controlled mating between two organisms, usually to obtain progeny of particular genotypes and phenotypes;
typically represented as shorthand such as PP x Pp
true-breeding
an organism homozygous for a particular trait;
A true-breeding will always "breed true" (produce offspring of its phenotype)
when bred to another organism true-breeding at the same gene locus.
hybrid
an organism that is heterozygous for a particular trait;
an organism produced by a cross of genotypically different parents.
monohybrid cross
a mating of two hybrids in which in which only ONE heterozygous locus is considered;
example: Aa x Aa
dihybrid cross
a mating of two hybrids in which in which two heterozygous loci are considered;
example: AaBb x AaBb
multihybrid cross
(trihybrid, tetrahybrid, etc.)
a mating of two hybrids in which in multiple heterozygous loci are considered;
example: AaBbCc x AaBbCc
parental (P) generation
the starting pair of individuals in a series of crosses
first filial (F1) generation
offspring of the P generation
second filial (F2) generation
offspring produced by the mating of two F1 individuals
back cross
a mating between a given individual and one of its parents
(or another individual of the parental genotype)
test cross
a mating between an individual expressing the
dominant phenotype of a given trait
with an individual that is homozygous recessive at the same locus
reciprocal cross
a mating of two phenotypic classes in which the parental sex is controlled
Test Cross: A Closer Look
A test cross is a mating between an individual expressing the dominant allele of a particular gene and an indivdual homozygous recessive at that same locus.
Offspring phenotypic ratios can reveal whether the dominant expressing parent is homozygous or heterozygous.
x
Reciprocal Cross: A Closer Look
A reciprocal cross is a mating of two phenotypic classes
while controlling for the sex of the parent.
The results can reveal whether the sex of the parent
has any influence on the inheritance of the trait in question.
x
The example on the right shows reciprocal crosses
between true-breeding parental fruitflies.
FIRST CROSS
P red-eyed female x white-eyed male
F1 progeny are 100% red-eyed (males and females)
RECIPROCAL CROSS
P white-eyed female x red-eyed male
F1 progeny are 50% red-eyed females
F1 progeny are 50% white-eyed males
This reveals that the trait in question is X-linked recessive.
A Punnett Square rapidly becomes unwieldy as more loci are added.
Fortunately, a few simple formulas allow quick predictions of genotypic and phenotypic ratios.
If n equals the number of traits/genes in question...
Number of F1 gamete types
2n
Proportion of F2 homozygous recessives
1/(2n)2
Number of different F2 phenotypes (complete dominance)
2n
Number of different F2 genotypes (or phenotypes, if no dominance)
