INHERITANCE

Cytogenetics

Prepared by: LGGRMT

3 Basic
Principles of
Heredity
 LAW OF DOMINANCE [Unit
Inheritance]
● Each trait is determined by two factors
(alleles), inherited from each parents.
● These factors exhibits a characteristic
dominant, co-dominant, or recessive
expression, and those that are dominant
will mask the expression of that are
recessive
 LAW OF SEGREGATION
● The two alleles of a gene are never
transmitted together from one parent to an
offspring. This means that, in humans, an
individual egg or sperm is formed with only
one allele of each gene
● Each of two inherited factors (alleles)
possessed by the parent will segregate and
pass during meiosis into separate gametes
(eggs or sperm), which will carry only one of
the factors.
 LAW OF SEGREGATION
● The two alleles of a gene are never
transmitted together from one parent to an
offspring. This means that, in humans, an
individual egg or sperm is formed with only
one allele of each gene
● Each of two inherited factors (alleles)
possessed by the parent will segregate and
pass during meiosis into separate gametes
(eggs or sperm), which will carry only one of
the factors.
 Genotype

Domains: subset of allele/individualized

Composed of: Allele

Produces: phenotype
 Phenotype

Domains: subset of allele/individualized

Composed of: Allele

Produces: phenotype
Alleles: different forms of a trait; one form of a gene

Example of traits: Example of Alleles

● Eye color - brown or blue

● Skin color - albino or pigmented
● Height - tall or short
● Hair texture - curly or straight
Alleles: different forms of a trait; one form of a gene
Where are genes and how are
they passed along?
Where are genes and how are
they passed along?
In figuring out what the
offspring “look” like…
● Dominant = the fully expressed
genes, has full control on the
phenotype
● Recessive = gene is completely
masked in the phenotype
Is there an easier way to figure out
what the offspring “look” like?
❏ Punnett square
 Is there an easier way to figure out
what the offspring “look” like?
❏ Punnett square
If the dominant trait is
brown eyes and the
recessive is blue eyes…
what are the
phenotype/s of the
offspring?
Sample problem:

● Mother (Aa) x Father (Aa)

- What are the genotypes of the offspring?
- If the dominant gene is attached earlobes and the
recessive gene is unattached earlobes, what are the
phenotype/s of the offspring?

Show me your seatwork :)

- What is the proportion of offspring with attached vs.
unattached earlobes?
Additional concepts about
inheritance that need to cover other
traits:
● A gene on one of the 22 pairs of autosomes - that is, the non-sex
determining chromosomes - is called autosomal gene.
● Similarly, a trait or diseases associated with that gene is autosomal
trait.
● Autosomal condition are the most common and are equally likely
to occur in males or females. Autosomal traits are further
classified as either dominant or recessive.
1.a AUTOSOMAL DOMINANT
INHERITANCE
● Autosomal dominant trait are those which as single copy of an
allele is enough for the trait to be expressed or shown in the
phenotype of the animal.
● Dominant conditions are those that are expressed in
heterozygous.
1.b AUTOSOMAL RECESSIVE
INHERITANCE
● Autosomal recessive traits require that the individual have 2
copies of the trait to express the phenotype
● The genes for autosomal-recessive traits are also located on the
autosomes, but the mutant, disease-causing allele; thus, if one
normal allele is present, it is usually sufficient to prevent any
expression of the disease.
2. X-LINKED INHERITANCE

● Females have two X chromosomes (XX) and males have an X and a

Y (XY)
● Smaller Y chromosome has only a very few genes as compared to
the larger X, X-linked inheritance is often referred to as sex-linked
inheritance.
● A distinguishing feature is the lack of male-to-male transmission,
because a father transmits only his Y chromosome, not his X, to
his sons
 X-linked dominant:
● Females are more frequently affected than
males
● Fathers cannot pass x-linked traits their
sons (no male-to-male transmission)
X-linked recessive
● Males are frequently affected than females
● Females w/ x-linked recessive disorder often have
affected males, but rarely affected females female in
each generation
● Only mother must be carrier of affected son (father
cannot pass x-linked trait to their sons)
3. MITOCHONDRIAL INHERITANCE

● Refers to the additional genes in cell’s mitochondria

● Because mitochondria are almost exclusively passed from
parent to child in the egg and not in the sperm, a hallmark
of mitochondrial inheritance is transmission from an
affected woman to all of her children.
4. CO-DOMINANT INHERITANCE

● Codominant alleles, both contribute to a phenotype.

Neither is dominant over the other.
● Control of the human blood type group system provides a
good example of co-dominant alleles.
X-LINKED DOMINANT INHERITANCE

● All daughters of an affected father will also be affected but

none of his sons will be affected (unless the mother is also
affected)
● In addition, the mother of an affected son is also affected
(but not necessarily the other way around)
X-LINKED RECESSIVE INHERITANCE

Expressed:
1. In males (who are necessarily hemizygous and,
2. In females who are homozygous for the gene mutation
Carrier females who have only one copy of the mutation do not
usually express the phenotype
 DEFINITIONS

❏ Gene = a sequence of nucleotides that represent a functional unit

of inheritance; a region of DNA that codes for a product either
RNA or protein
❏ Chromosome = a highly ordered structure composed of DNA and
proteins that carries the genetic information. In humans, there are
46 chromosomes ordered in pairs
❏ Autosome: all chromosomes other than the X and Y
chromosomes which are designated the sex hormones
 DEFINITIONS

❏ Locus: position of a gene on a chromosome.

❏ Allele: an alternative form of gene occupying the same locus. An
allele may be the result of a mutation. There is a maximum of two
alleles per diploid chromosome complement (one allele per
chromosome), but multiple alleles may exist with a population.
 DEFINITIONS

❏ Mutation: a permanent change in the sequence of genomic DNA.

this may manifest at both the molecular and cytogenetic levels.
- Not all mutations are negative events.
- Many benign (i.e., sickle cell trait in counties with significant risk of
malaria)
- An individual with constitutional mutation (i.e., a mutation present
in every cell of the body) may pass that mutation on to their
progeny by germ-line transmission.
 DEFINITIONS

❏ Mutation:
- In some cases, notably cancer, an acquired mutation may rise in a
single somatic cell. This mutation will be limited to the clone
composed of the mitotic products of the original mutant cell and
will not be transmitted to progeny of the individual.
- In rare instances of gonadal mosaicism, a de novo acquired
mutation may arise in the gonads resulting in a mixed population
of normal and mutant gametes.
 DEFINITIONS
❏ Mutation:
- Progeny receiving the new mutation may display a phenotype not
present in either parent
❏ Karyotype: the chromosome constitution of an individual. Also, a
figure showing the arrangement of the paired chromosomes from
a cell in a standard sequence.
❏ Diploid: the presence of two copies of each unique chromosome
per cell. In humans, the chromosomes occurs in pairs and the
diploid (2n) number is 46
 DEFINITIONS
❏ Haploid: one copy of each unique chromosome. In humans, the
gametes are haploid (N=23)
❏ Homozygous: both alleles at a locus are the same. (In ABO
system, an AA complement represent homozygosity)
❏ Heterozygous: the two alleles at a locus are different. (In ABO
system, an AO complement represent heterozygosity)
❏ Hemizygous: the presence of only one chromosome or
chromosome segment rather than the usual two; applies to males
with a single X chromosome
 DEFINITIONS
❏ Genotype: the genetic constitution of an individual or organism
(i.e., what alleles are present). (In ABO system, AA, AO, BB, BO, AB,
an OO are genotypes)
❏ Phenotype: the appearance of an individual that results from the
interaction of environment and genotype. (In ABO system, A, B,
and O blood types represent the phenotypic expression of the
alleles for a given individual.)
❏ Independent assortment: random assortment of chromosomes
(paternal and maternal) in the gametes; 50:50 chance of inheriting
a given chromosome from one parent
 DEFINITIONS
❏ Linkage: the presence of two or more genes on the same
chromosome that tend to be inherited together.
❏ Crossing over: the physical exchange of genetic material between
homologous chromosomes.
❏ Recombination: the generation of new allelic combinations on
chromosomes, usually by crossing over.
❏ Mitosis: somatic cell division in which the DNA replicates and is
evenly distributed to two equal daughter cells
 DEFINITIONS
❏ Meiosis: cell division in the gonads that produces the gametes. A
single DNA replication is followed by two cell division. A single
DNA replication is followed by two cell divisions which reduced the
total DNA content of a cell from 2N to N. recombination occurs to
increase genetic diversity diversity within a population.
❏ Nondisjunction: failure of chromosomes or chromatids to
separate to opposite poles in cell division. Usually results in one
too many or one too few chromosomes in a cell.