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Unit 3 – Genetic Continuity
Crosses with answers- http://mail.esdnl.ca/~patrick_wells/biology/genetics/genetics.html
Mendelian Genetics (patterns of inheritance)
Genetics is the study of patterns of inheritance. In humans, physical characteristics of a person
(phenotype) are due to heredity (genotype) and environmental influences. Mendel studied
patterns of inheritance using pea plants. He had two main laws aside from his Law of
Dominance.

Law of Segregation –
alleles split during
gamete formation
(proved by monohybrid
cross 3:1 ratio – do a
sample on the side Tt x
Tt)

Law of Independent
Assortment - alleles for
different traits split
independent of each
other during gamete
formation (proved by his
dihybrid cross 9:3:3:1
ratio).
F1 TTBB x ttbb
F1 Results?
F2 Results?
In earlier times, scientists thought that inherited traits were "blended" in the offspring. Mendel
discovered traits were passed on for each characteristic with one copy of that trait coming from
each parent. For many characteristics there was more than one version of a trait (we now call
these alleles). For each gene an organism has two alleles one inherited from each parent.
We know about meiosis – this cell division produces monoploid gametes (eg: sperm and egg)
and the allele pairs are separated from each other at this time (Mendel’s segregation).
Fertilization reunites allele pairs to make the gene complete again - the fertilized egg has one
allele for each gene and the sperm has one for each gene. This interaction makes of alleles is
what makes you and genetics so interesting!
As Mendel discovered alleles are usually DOMINANT or RECESSIVE. A dominant allele is
fully expressed when present. A recessive allele is fully masked and is only expressed in
homozygous form.
Recessive phenotype is also a genetype – this is useful in solving problems!
Mendel was able to come to this conclusion through performing monohybrid crosses (looking at
a single trait and following its expression through F1 and F2 generations). You should practice
your crosses. Also use your memory tricks.
Do some crosses: http://mail.esdnl.ca/~patrick_wells/biology/genetics/genetics.html
Terms

diploid cell: A somatic cell in human. In humans we have 23 maternal and 23 paternal
chromsomes giving us a diploid number of 46. (2n=46)

Males – 44 autosomes and XY sex chromosomes Females 44 autosomes and XX sex
chromosomes

monoploid cell: Sex cell in humans. Also called haploid as these cell have only half the
normal number of chromosomes (sperms and eggs). A monoploid human egg or sperm has
23 chromosomes. (1n=23)

gametes: Sex cells, either eggs or sperm, these cells are always monoploid.

Homozygous or Pure means having two identical alleles of a gene. TT or tt

Heterozygous or Hybrid is when you have two different alleles of a gene. Tt

A carrier is hybrid and has a recessive allele of a gene that does not have an effect on their
phenotype.

A test cross is testing an unknown dominant individual by crossing it with a known
homozygous recessive. If ANY recessive offspring result the unknown must have been
heterozygous.
The genotype is the genes possessed by an organism. The phenotype is the physical
characteristics of an organism. A dominant allele is an allele that has the same effect on the
phenotype whether it is present in the homozygous or heterozygous state (upper case letters). A
recessive allele is an allele that only has an effect on the phenotype when present in the
homozygous state (lower case letters).
Incomplete dominant alleles: Alleles that are simultaneously expressed in an altered form –
snap dragons hybrids are pink not white or red. (RR – red, WW – white and RW – pink)
Codominant alleles are pairs of alleles that both affect
the phenotype when present in the heterozygous state.
Alleles that are simultaneously fully expressed in the
heterozygous condition (AB blood type). These can be
symbolized by upper case letters. We use these
a a a o
symbols I I I I for type A blood. The ABO blood
groups are an example of multiple alleles of a single
a
gene because this gene exists in three allelic forms: I ,
Ib and Io(or some teachers us i).The Ia and Ib is
o
dominant to the I allele. Type O will only be
expressed in the homozygous form; when combined
with A or B alleles it will not be expressed. For
example, a person with both the A and B alleles,
carries AB type blood. Both blood group A and B are
fully expressed. Some genes have more than two
alleles (multiple alleles). Blood groups are the best
examples of multiple alleles and co-dominance – know
how to do these crosses!
Genes are on Chromosomes! (Sutton suggested this theory)
The chromosome theory states that chromosomes are linear sequences of genes. The unifying
theory states that inheritance patterns may be generally explained by assuming that genes are
located in specific sites on chromosomes (recall that each gene makes a specific protein). Sutton
was the first to point out that chromosomes obey Mendel's rules—the first clear argument for the
chromosome theory of heredity. Sutton worked with grasshopper chromosomes, and it was in this
paper that he showed that chromosomes occur in distinct pairs, which segregate at meiosis.
Linked genes are on the same chromosome – they do not follow the Law of Independent
Assortment. The discovery that genes were on chromosomes made the Law of Independent
Assortment invalid for genes on the same chromosome. Refers to meiosis – tetrad separation is
the reason for independent assortment – but it only works if the genes that are assorting are on
different chromosomes
Sex in humans is determined by two chromosomes, called X and Y (X is bigger than Y in
karyotypes). All males (♂) have one X chromosome and one Y chromosome. Females (♀) have
two X chromosomes. In meiosis, therefore, females can only produce gametes with an X
chromosome, while males can produce gametes with either an X or a Y chromosome. The male's
gametes, then, are those that decide gender: the child can have XX (female) or XY (male)
chromosomes depending on what it receives from its father. This is another example of
segregation.
Color-blindness and hemophilia are probably
the most common examples of sex-linked
traits in humans. Both are due to a recessive
sex-linked allele on the X chromosome. They
are more common in males than females.
Obviously a recessive X-linked gene will
only be expressed in the homozygous form,
as this is part of the definition of recessive
genes. Therefore, if an X-linked recessive
alleles is present in a male, it will always be
expressed, as this is the only X gene the male
possesses. However, females have two X
genes, only one of which is actually
expressed. The other is bound up in an
inactive structure known as a Barr body.
Therefore if the X chromosome is the one
bound in the Barr body, its recessive alleles
are not expressed, and the female may be a
carrier without displaying any effects.
Morgan suggests sex linkage in fruit fly eye
colour – cross.
Polygenic traits have two or more genes. As opposed to monogenic such as blood type – you
have a distinct blood type. Skin color and height are polygenic. It is by determined by a number
of genes (so we have a range of height in humans not tall and short people depending on how
many alleles you have for “tallness”).
You must be able to analyze and interpret models of human karyotypes (remember the lab on
Karyotypes). If you can’t perform crosses for dominant, sexlinked, codominant and incomplete
dominant traits you will have great difficulty with the remainder of this course!
Molecular Genetics (Genetics of the DNA molecule) First the history ………..
Levene analyzed the components of the DNA molecule. He found it contained four nitrogenous
bases: cytosine, thymine, adenine, and guanine; deoxyribose sugar; and a phosphate
group.
Frederick Griffith was able to induce a non-pathogenic (harmless) strain of the bacterium
Streptococcus pneumoniae to become pathogenic (deadly). Griffith referred to a transforming
factor (we know it as DNA) that caused the non-pathogenic bacteria to become pathogenic.
Oswald Avery, Colin MacLeod, and Maclyn McCarty revisited Griffith's experiment and
concluded the transforming factor was DNA. Their evidence was strong but not totally
conclusive. The then-current favourite for the hereditary material was protein; DNA was not
considered by many scientists to be a strong candidate.
Hershey and Chase sought an answer to the question, “Is it the viral DNA or viral protein coat
(capsid) that is the viral genetic code material which gets injected into a host bacterium cell?
With the blender experiment they conducted show that DNA was the molecule of heredity.
Watson and Crick gathered all available data in an attempt to develop a model of DNA structure.
Franklin took X-ray diffraction photomicrographs of crystalline DNA extract, the key to the
puzzle. The data known at the time was that DNA was a long molecule, proteins were helically
coiled (as determined by the work of Linus Pauling), Chargaff's base data (A to T and G to C
ratios), and the x-ray diffraction data of Franklin and Wilkins.
Barbara McClintock was the first scientist to predict that transposable elements, mobile pieces of
the genetic material (DNA), were present in eukaryotic genomes.
Structure and Replication
The sides of the ladder of DNA consist of alternating phosphate groups and
deoxyribose (a sugar). The two sides are antiparrallel, meaning that the sugar
and phosphates are running in opposite directions (anti parallel). DNA is the
primary molecule of heredity and controls the production of proteins in all
organisms. RNA is a molecule that assists DNA conduct its primary function
– making proteins or polypepetides.