1. Patterns of inheritance
I. Observation:
Offspring resemble their parents
II. Conclusion:
Offspring inherit their physical characteristics from their parents
III. The patterns of inheritance are not always obvious
A. Sometimes, traits in offspring appear to be a blend of
the traits in the parents
B. Other times, the traits in the offspring appear to be like one
or the other parent
C. Yet other times, the traits seem nothing like the parents. Some
traits may have “skipped” one or more generations.
IV. So what is going on?

2. Patterns of inheritance
Terminology:
Genes: Segments of the DNA on chromosomes that code
for a specific protein
Locus (loci): The specific physical location of a gene on the chromosome
Homologous chromosomes: Chromosomes that carry the same genes.
Since most cells are diploid, they have a set of two
chromosomes and therefore two copies of each gene.
Alleles: Although homologous chromosomes carry the same “type” of
genes, the specific sequence of the same gene on the two
homologous chromosomes may be slightly different.
The different forms of the same gene found on the different
chromosomes are referred to as alleles.

3. Patterns of inheritance (Mendel’s experiments explained)
Parental generation are true breeding:
they are homozygous, which
means that they have the exact
same gene (i.e. allele) form for color
on their homologous chromosomes.
The first generation looks like one of the parents.
Genetically however, it is heterozygous, which
means it has two different allele forms, one from
each parent.
The first generation flowers look purple, because the purple allele is dominant,
And the white allele is recessive.
Terminology:
The purple parent is referred to as homozygous dominant.
The white parent is referred to as homozygous recessive.
The purple offspring is referred to as heterozygous.

4. Patterns of inheritance
Terminology (cont.):
For each organism, the general
appearance is referred to as the
phenotype.
For each organism, the genetic
makeup of the alleles is referred to as the
genotype.

5. Patterns of inheritance
Self fertilization of the F1 generation will result in a second generation where
¾ of the offspring are purple and ¼ of the offspring are white.
Mendelian genetics will allow us to explain what happens here, and we will be
able to mathematically predict the outcome of various crosses.

6. Patterns of Mendelian inheritance – a closer look
In the parental generation, heterozygous
individuals produce gametes all with the
same allele type.
(note that the dominant alleles is typically
marked with a capital letter symbol, and
the recessive allele is marked with a lower
case letter symbol)
The offspring generation however
is heterozygous and will therefore
produce gametes that have different
alleles.
The ratios of the alleles in the gametes
are predictable; in this case 50:50

7. Patterns of Mendelian inheritance – a closer look
The pattern by which the gametes
fertilize each other will determine
the phenotype of the next generation.
If our sample sizes are large enough,
then we can actually mathematically
predict both the genotypic and the
phenotypic outcome of the next
generation.

8. Patterns of inheritance – The Punnett Square
If we know that we are self fertilizing a heterozygous
plant, then we know that ½ of the eggs and ½ of the
sperm will have the dominant (P) allele, and that
the other half will have the recessive (p) allele.
Next, since we know that during fertilization,
the genetic material of the egg and the sperm
combine, then we can simply cross multiply
our alleles to come up with the genotype of the
next generation; in this case:
¼ PP : ½ Pp : ¼ pp
Now that we have predicted the genotype,
We can use our knowledge of dominant
Versus recessive genes to predict the
Phenotypic ratio of our offspring; in this case:
¾ purple : ¼ white

9. Patterns of inheritance – The Punnett Square
By simply looking at the pea plant flowers,
we may or may not be able to predict the
genotype.
However, since the patterns of inheritance
are predictable, we can use simple experimentation
and Punnett squares to establish genotypes.

10. Patterns of inheritance
Mendel went beyond considering
just one trait though. He considered
a number of traits and how combinations
of these traits are inherited.

11. Patterns of inheritance
If the alleles being studied are found
on different chromosomes, then their
distribution will be independent of each
other. In these cases, Punnett square
analysis can still be a powerful tool in
predicting and studying patterns of
inheritance.
At this point, you should be able to pick
any two genotypes (consider the possibilities
from the chart to the right), cross them,
and be able to predict the genotypic as well as
phenotypic outcome of the next generation.

12. Patterns of inheritance
For multiple genes, the patterns of inheritance are predictable because of the
independent (or random) assortment of alleles

13. Patterns of inheritance
Punnet squares can be used to study inheritance pattern of multiple traits
that are not on the same chromosome – i.e. are independently inherited.
Genes that are located on the same chromosomes however do now follow the
same pattern because they are inherited together and are said to be “linked”.
Patterns of inheritance for linked genes typically follow the patterns of a
single gene.

14. Patterns of inheritance
Patterns of inheritance for linked genes typically follow the patterns of a
single gene.
In some cases however, linked genes can be separated by recombination
due to crossing over:
>>>>
These recombinations will cause variation on the basic patterns of inheritance
predicted by Mendelian genetics and Punnett squares.

15. Patterns of inheritance – Sex determination in mammals
In mammals, XX offspring are female
whereas XY offspring are male. This pattern
May be different for other organisms…
The Y chromosome typically carries just a
few genes, whereas the X chromosome may
carry many genes that may or may not have
anything to do with the sexual outcome of the
animal.
Genes that are on one sex chromosome but
not on the other are said to be sex-linked.
What do you think would be the pattern of
inheritance in sex-linked genes?

16. Variations on the Mendelian Theme
1. Alleles may display incomplete dominance – heterozygous individuals may have
phenotypes that are intermediates between phenotypes of the homozygotes.

17. Variations on the Mendelian Theme (cont.)
2. A single gene may have multiple alleles, some of which may be dominant over
Others, whereas other alleles may be codominant (e.g. human blood groups)

18. Variations on the Mendelian Theme (cont.)
3. Some traits may be influenced by several genes – polygenic inheritance (e.g. human eye or skin color)

19. Variations on the Mendelian Theme (cont.)
4. Single genes may also have multiple effects on phenotype – they may be
regulatory proteins that affect other genes.
5. The environment can influence the expression of some genes.

20. Patterns of Mendelian inheritance in humans
Mendelian inheritance can be seen in many of the phenotypic characteristics observed
in humans – we will consider some of these themes in the laboratory section of this lecture.