BL414 Genetics Spring 2006
Lecture 2 Outline
January 23, 2006
Chapter 2:
2.5 Important Terms:
Gene: element of heredity
Alleles: alternative forms of a given gene
Chromosomes: condensed form of chromatin; houses the genetic material,
observable during mitosis, composed of DNA tightly coiled around histone
proteins
Humans have 46 chromosomes, 2 homologous sets of 23 different
chromosomes
Locus: the position of a gene on a chromosome, also indicates a single gene –
“genetic locus”
Genotype: the genetic constitution of an individual organism – often used in
reference to a particular gene
Homozygous: individual has 2 of the same alleles for a particular gene, at
a particular locus
Heterozygous: individual has 2 different alleles for a given gene
Phenotype: the observable traits in an individual caused by expression of the
individual’s genotype
These traits can be biochemical, behavioral, physical, etc. A molecular
phenotype could also be observed using molecular biology techniques.
There is not always direct correspondence between genotype and
phenotype.
Some phenotypes which don’t seem to fit Mendelian genetics are caused
by more than one gene or by the environment.
2.6-2.7: There are molecular biology techniques for observing differences in
human genetics sequences at the DNA level. This is useful for understanding the
human genome and human genetic diseases.
DNA polymorphism: differences in the DNA sequences of individuals
SNP: single-nucleotide polymorphism: a common variation in nucleotide
sequence at a particular site in the human genome, this doesn’t include very rare
sequence differences that are in less than 1% of the population. 500,000 SNP’s
have been mapped – this is useful because it gives us some human alleles to keep
track of, even though we might not yet know the genes or their functions in a
particular stretch of the genome
RFLP: restriction-fragment length polymorphism: a difference in DNA
sequence that occurs within a restriction site – this makes it easy to study because
it is observable by a restriction digest of the DNA in question
Chapter 3: Transmission Genetics: the Principle of Segregation
Transmission genetics: the study of inheritance of traits
Mendelian genetics: follow the principles first defined by Mendel, synonymous
with transmission genetics
3.1 Morphological and Molecular Phenotypes
Terms:
Wildtype: the nonmutant allele of a gene – usually the most common allele
found in nature, or in the organism being studied
Dominant allele: is expressed whether heterozygous or homozygous
Recessive allele: expressed only when homozygous
“Dominant” or “recessive” can also refer to the trait itself.
Transposable element: a sequence of DNA that is able to move around in
the genome, sometimes inserting into other genes possibly causing disruptions
Codominant: both alleles in a heterozygote are expressed
Mendel used peas in his genetic studies – looked at various morphological traits:
seed shape, seed color, flower color, pod shape, and others.
*Mendel’s genius was in that he chose simple, mutually exclusive traits and he
studied the inheritance of these traits.
For example, Mendel studied the gene for wrinkled seeds
Phenotype: round vs. wrinkled
Gene product (we now know): SBEI, starch branching enzyme I, which is
involved in the synthesis of the branched-chain starch amylopectin, which
keeps seeds smooth!
Alleles: W or w
Wildtype allele: W
Wildtype phenotype: round
phenotype allele wildtype Gene
Molecular
or
product basis
mutant
round
W
wildtype SBEI
Normal
wrinkled
w
mutant
absent
Insertion
into the gene
dominant
or
recessive
W=
dominant
w=
Genotype
WW, or
Ww
ww
that disrupts
expression of
the SBEI
protein
recessive
3.2 Segregation of a single gene
Mendel’s studies came up with the important and essential concept of the
segregation of genes. How did he do it?
His pea plants worked well for these experiments because they self-pollinate
unless Mendel intentionally surgically cross-pollinated (or “outcrossed”) them
with another plant of his choosing.
If a pea plant was homozygous, and was allowed to self-pollinate, its offspring
would also remain homozygous. E.g. ww  ww, or WW  WW. In this way,
another name for homozygous plants is “true-breeding”  they will always
produce identical offspring.
Breeding pea plants from parents with different traits for a given gene produced
hybrid offspring. For a single trait, this is called monohybrid, for two traits:
dihybrid, for three traits, trihybrid, etc.
P1 generation: the parents
F1 generation: the offspring of the parents. In Mendel’s experiments, The F1
generation all had the dominant phenotype.
Reciprocal crosses: Male of one trait mated to female with a different trait, and
vice versa. For the wrinkled gene, reciprocal crosses always have the same result.
For example:
Cross A: Male (pollen from) WW x Female (Stigma) ww  Ww
Cross B: Male (pollen from) ww x Female (Stigma) WW  Ww
F2 generation: the offspring of mating F1 individuals to each other. In Mendel’s
experiments, the F2 generation of the hybrid crosses always had a phenotypic
ratio of 3:1 of dominant: recessive.
In explaining and understanding the 3:1 ratio, we come to the heart of
“Mendelian genetics.” The ratio is explained by looking at what happened to the
genes during the mating and production of offspring. Remember that at the time
of Mendel’s studies, nothing was known about genes or chromosomes or DNA,
all Mendel had was the numbers of offspring of various phenotypes.
The principle of segregation of alleles
Key features of single gene genetics:
1. Genes come in pairs – an individual has two alleles of each gene.
2. For a given gene, the alleles can be identical (homozygous) or different
(heterozygous).
3. Each gamete (reproductive cell) from an individual has only one of the
two alleles.
4. In the formation of gametes, each one is equally likely to get one or the
other allele.
5. The union of male and female gametes is a random process that reunites
the alleles in pairs.
Points 3 and 4 are essentially the principle of segregation(also called the
Mendel’s first law). More formally (from your book), the principle holds that:
“In the formation of gametes, the paired hereditary determinants separate
(segregate) in such a way that each gamete is equally likely to contain
either member of the pair.”
This segregation of alleles yields the results observed by Mendel and holds
true for the genetics of all higher organisms. (Plants, animals, humans)
Punnett square: a diagram of the alleles contained in the gametes of a
particular genetic cross, which allows the calculation of the offspring
genotypes and the projected ratios of genotypes and phenotypes.
Verification of segregation:
Testing the genetic makeup of an organism:
Self-fertilization of the F1 progeny gave results that supported the principle of
segregation. Round F1 offspring that were self-fertilized yielded plants that
either gave only round F3 seeds (WW F2) or plants that were both round and
wrinkled in the 3:1 ratio (Ww F2), and the WW:Ww F2’s were in a ratio of 2:1.
Testcross: a cross between an organism that is heterozygous for one or more
genes (e.g. Ww), and an organism that is homozygous for the recessive allele
(ww). Ww x ww  expect offspring to be 1:1 dominant:recessive (50% Ww,
50% ww)
Backcross: a cross between a heterozygous organism (Ww) and one of the
parental genotypes (WW or ww)