Important
words and concepts from Chapter 14, Campbell & Reece, 2002
(1/29/2005):
by Stephen T. Abedon (abedon.1@osu.edu)
for Biology 113 at the Ohio State University
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Course-external links are
in brackets Click [index] to access site index Click here to
access text’s website Vocabulary
words
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(1) 
Chapter title: Mendel and the Gene Idea
(a)
“The best way to gain an understanding of genetics is to work with it.
The fundamental principles discussed (below) will become clear to you, and you
will grasp them more surely, if you carefully think through . . . problems
which illustrate the various patterns of inheritance…” (Keeton, 1980,
Biological Science third edition, W.W. Norton & Company, p. 621)
(b)
[Mendel and the gene idea
(Google Search)] [Genetic Science Learning
Center] [Genetics (a journal) (The Genetics Society of America)] [Three neglected advances in
classical genetics] [index]
(2)
Gene [genotype]
(a)
A gene is a discrete heritable unit
(b)
Genes are found at specific loci on chromosomes
(c)
(loci mapped to human chromsome number 3
are shown to the right; M b means megabase and cM means centimorgan) à
(d)
Most genes generate traits (phenotype) via
their coding for the synthesis of proteins
(e)
[gene (Google Search)] [index]
(a)
The sequence of nucleotides on chromosomes represents genotype
(b)
Genotype is subdivided into discrete, heritable units called genes
(c)
Genotype is what is passed from parent to offspring on chromosomes
(d)
[genotype (Google Search)] [index]
(a)
Phenotype is specified by genotype
(b)
Phenotype is the stuff that you can see and measure about an organism
(c)
For example, height, color, number of legs, ability to smell, etc.
(d)
Phenotype imperfectly maps onto genotype
(e)
That means that there may exist numerous genotypes for any one
phenotype
(f)
An example of such imperfect mapping are the phenotypes associated with
dominant and recessive alleles
(g)
See Figure 14.5, Genotype
versus phenotype
(h)
[phenotype (Google Search)] [index]
(5)
Allele [genotype]
(a)
The genes found at a given locus may differ between homologous chromosomes
(b)
Often the differences represent only one or a few nucleotides
(c)
Two genes that are found at the same locus of a homologous pair of chromosomes, that differ in nucleotide sequence, are referred
to as different alleles
(d)
Different alleles may or may not elicit (code for) different phenotypes
(e)
When a gene has a dominant form and a recessive form, these forms are
considered to be separate (i.e., different) alleles which give rise to distinct
phenotypes (although it is also possible for two distinct alleles to give rise
to the same phenotype)
(f)
A diploid organism has up to two alleles present in their genome
per locus, or as few as one
(g)
See Figure 14.3, Alleles, alternative
versions of a gene
(h)
[allele (Google Search)] [index]
(a)
A character is a heritable feature (phenotype) of an
individual
(b)
A character can vary
(c)
For example, hair color is a character as is ear size,
etc.
(d)
See Table 14.1, The results
of Mendel’s F1 crosses for seven characteristics in pea plants
(e)
[genetic character (Google Search)] [index]
(7)
Trait [phenotype]
(a)
A variant of a character is a trait
(b)
Thus, we have genes that are responsible for a given character
(c)
And we have different alleles of that gene which are responsible for
different traits associated with that character
(d)
For example, a purple flower may have an allele which codes for a
purple dye at the flower-color loci
in the plant's genome; flower color
is the character and purple is the trait
(e)
See Table 14.1, The results
of Mendel’s F1 crosses for seven characteristics in pea plants
(f)
[genetic trait (Google Search)] [index]
(a)
In diploid organisms, phenotype typically imperfectly maps onto genotype
(b)
This is in part because diploid organisms (but not haploids) can possess up to two alleles at a given locus
(one on each homologue)
(c)
See Figure 14.3, Alleles, alternative
versions of a gene
(d)
Each of these alleles, whether identical or different, will interact to
produce a trait
(e)
The interaction between non-identical alleles results in interesting
non-correspondences between genotype and phenotype
(f)
[diploidy, diploid (Google Search)] [index]
(9) Segregation of
alleles (Mendel’s law of segregation) [genotype]
(a)
Note that during meiosis the allele found on one
homologue will segregate from the homologous allele found on a
the other homologue
(b)
This is cytogenetical basis for Mendel’s law of segregation
(c)
See Figure 14.4, Mendel’s
law of segregation
(d)
[segregation of alleles,
Mendel's law of segregation
(Google Search)] [index]
(10)
True breeding [genotype]
(a)
Diploidy also results in interesting patterns of inheritance
(b)
The simplest pattern results in all offspring (and their offspring,
etc.) always resembling the parents
(c)
In such a situation we can infer that the parents together possessed
only a single allele at the locus in
question
(d)
This would be a total of four identical alleles between the two original
parents
(e)
Such characters are said to be true breeding because they fail to vary
through the generations
(f)
["true breeding"
(Google Search)] [index]
(11)
Homozygosity (homozygote)
[genotype]
(a)
True breeding stems from homozygosity
(b)
Homozygosity simply refers to a lack of non-identity (they’re the same)
of the two alleles found at a given locus within
a diploid
individual
(c)
Such an individual is said to be a homozygote
(d)
[homozygosity, homozygote (Google Search)] [index]
(12)
Heterozygosity (heterozygote)
[genotype]
(a)
An individual who possesses two different alleles at a given locus is
said to be a heterozygote
(b)
Note that the two alleles defining a heterozygote will segregate into different gametes such that 50% of gametes will posses one allele and
the rest (50%) of the gametes will posses the other allele
(c)
[heterozygosity, heterozygote (Google Search)] [index]
(13) Homologous allele [genotype]
(a)
A homologous allele is an allele that is found at the same locus on
a different, homologous chromosome
(b)
That is, a gene can consist of various alleles (up to 2 in
a diploid
individual), each of which within the same individual is considered homologous
to each of these other alleles
(c)
Thus, at a given locus a flower may contain an allele that codes for
the trait
purple flowers; the homologous allele, found on a homologous chromosome, might
code for white flowers (or purple flowers or whatever), but not plant height,
etc. (unless the gene has pleiotropic effects)
(d)
[homologous alleles
(Google Search)] [index]
(14) Dominance
relationships (dominant
alleles) [phenotype]
(a)
Some alleles are capable of expressing a trait at the
expense of a second, homologous allele within a heterozygote
(b)
Such alleles (the former) are said to display dominance
(c)
When we abbreviate alleles, typically the dominant allele is
capitalized (e.g., A)
(d)
[dominance relationships
genetics, dominant alleles (Google Search)] [index]
(15)
Recessive alleles [phenotype]
(a)
An homologous allele that fails to have an impact
on phenotype
when paired with a dominant allele is said to be recessive
(b)
When we abbreviate alleles, typically the recessive allele is
written in lower case (e.g., a)
(c)
[recessive alleles (Google Search)] [index]
(16)
Homozygous recessive [phenotype]
(a)
An individual who possesses only a recessive
allele at a given locus is
said to be homozygous recessive
(b)
That is, at that locus the individual’s genotype would be aa where a is the abbreviation for the recessive allele
(c)
[homozygous recessive
(Google Search)] [index]
(17)
Homozygous dominant [phenotype]
(a)
An individual who possesses only the dominant allele at
a given locus is
said to be homozygous dominant
(b)
That is, at that locus the individual’s genotype would be AA where A is the abbreviation for the dominant allele
(c)
[homozygous dominant
(Google Search)] [index]
CROSSES
(a)
A cross is a mating between two individuals
(b)
We abbreviate the occurrence of a cross as an x found between two
genotypes, e.g., AaBBcc x aaBBcc is a cross between two
individuals where we are keeping track of alleles found at
three different loci
(locus A, locus B, and locus C)
(19)
Monohybrid cross [genotype]
(a)
A monohybrid cross is a mating between two individuals who
we are scoring for variation in a single character controlled by a single locus
(b)
For example, flower color in peas
(c)
An example of a monohybrid cross could be abbreviated as Aa x Aa
(d)
Note that each individual participating in a monohybrid cross is heterozygotic
at the locus in question
(e)
See Figure 14.4, Mendel’s
law of segregation
(f)
[monohybrid, monohybrid cross, monohybrid cross problems
(Google Search)] [index]
(a)
P stands for parental
(b)
The parents are the first generation to be crossed (i.e.,
mated)
(c)
In an experimental breeding program this first generation is called the
P generation
(d)
See Figure 14.4, Mendel’s
law of segregation
(e)
["P generation",
parental generation
(Google Search)] [index]
(a)
The F1 generation is the product (the offspring) of the
parental cross (P generation)
(b)
F stands for filial
(c)
See Figure 14.4, Mendel’s
law of segregation
(d)
[F1 generation, first filial (Google Search)] [index]
(a)
The F2 generation is the product (the offspring) of the
interbreeding of the F1 generation
(b)
See Figure 14.4, Mendel’s
law of segregation
(c)
[F2 generation, second filial (Google Search)] [index]
(23) Cross between homozygous dominant and homozygous recessive [genotype & phenotype]
(a)
See Figure 14.4, Mendel’s
law of segregation
(b)
AA x aa = P generation
(c)
Aa = F1 generation
(d)
AA + Aa + Aa + aa = F2 generation
(e)
Say A is a dominant
allele (e.g., codes for a purple flower color)
(f)
Say a is a recessive allele (e.g., cods for a white flower color)
(g)
Then the F1 generation will have only purple flowers
(h)
The F2 generation will have three purple flowers for every
white flower