Inheritance
Inheritance is the passage of hereditary traits from one generation to the next. It is the process by
which you acquired your characteristics from your parents and transmit some of your traits to your
children. The branch of biology that deals with inheritance is called genetics.
Genotype and Phenotype
The nuclei of all human cells except the gametes contain 23 pairs of chromosomes, the diploid number
(2n). One chromosome in each pair came from the mother and the other came from the father. Each of
these two homologous chromosomes contains genes that control the same traits. Alternative forms of a
gene that code for the same trait and at the same location on homologous chromosomes are called alleles.
A mutation is a permanent heritable change in an allele that produces a different variant of the same trait.
A chart which shows the possible combinations of gametes from two parents is called a Punnett square.
In constructing a Punnett square paternal alleles are written at the left side, maternal alleles are written at
the top. The four filled spaces on the chart show how the alleles can combine in the zygotes formed by
the union of these sperm and ova to produce three different combinations of genes, or genotypes. From
the Punnett square 25% of the offspring will have the homozygous dominant genotype, 50% will have the
heterozygous dominant genotype and 25% will have the homozygous recessive genotype. The allele that
dominates or masks the presence of another allele and is fully expressed is dominant allele. The trait
expressed is called the dominant trait. The allele whose presence is completely masked is said to be a
recessive allele. The trait it controls is called the recessive trait.
The symbols for the dominant alleles are written in capital letters and for the recessive alleles are
written in lower case letters. A person with the same alleles on homologous chromosomes is
homozygous for the trait. An individual with different alleles on homologous chromosomes is
heterozygous for the trait.
Phenotype refers to how the genetic makeup is expressed in the body; it is the physical or outward
expression of a gene. The heterozygote individual has a different genotype from the homozygote
individual, but both have the same phenotype. Heterozygous individuals who carry a recessive gene
but do not express it can pass the gene on to their offspring. Such individuals are called carriers of the
recessive gene.
Most genes give rise to the same phenotype whether they are inherited from the mother or the father.
In a few cases, the phenotype is different depending on the parental origin of the genes. This is called
genomic imprinting.
Alleles that code for normal traits usually dominate over those genes that code for abnormal traits.
Dominant alleles for severe disorders are usually lethal and cause death of the embryo.
Errors in cell division, called non-disjunction, results in an abnormal number of chromosomes. In this
situation, homologous chromosomes (during meiosis I) or sister chromatids (during anaphase of
mitosis or meiosis II) fail to separate properly. A cell from which one or more chromosomes has been
added or deleted is called aneulpoid. A monosomic cell (2n-1) is missing a chromosome; a trisomic
cell (2n+1) has an added chromosome. Down syndrome is caused by trisomy of chromosome 21. The
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non-disjunction usually occurs during meiosis. A small number of Down syndrome cases result from
non-junction during mitosis in early embryonic development.
Translocation is an error in meiosis. In this case, two chromosomes that are not homologous break and
interchange portions of their chromosomes. If there is no loss of genetic material when the meiosis
occurs the individual who has the translocation may be normal. However, some of this person’s
gametes formed during meiosis may not contain the correct amount and type of genetic material. Some
Down syndrome cases result from a translocation of part of chromosome 21 to chromosome 14 or 15.
The individual who has this translocation is normal and do not know that they are “carriers” for this
condition. When such individuals produce gametes some gametes end up with a whole chromosome 21
plus another chromosome with the translocated fragment of chromosome 21. Upon fertilization, the
zygote has three, rather than two, copies of that part of chromosome 21.
Variations on Dominant – Recessive Inheritance
Dominant-Recessive Inheritance
In this type of inheritance the dominant and recessive alleles interact. The phenotypic expression of a
particular gene may be influenced not only by which alleles are present, but also by other genes and by
the environment. Most inherited traits are influenced by more than one gene and most genes can
influence more than one trait.
Incomplete Dominance
In incomplete dominance, neither member of a pair of alleles is dominant over the other, and the
heterozygote has a phenotype intermediate between the homozygous dominant and the homozygous
recessive phenotypes. An example of incomplete dominance in humans is the inheritance of sicklecell disease (SCD). People with the homozygous dominant genotype HbAHbA form normal
hemoglobin; those with the homozygous recessive genotype HbSHbS have sickle-cell disease and
severe anemia. Those with the heterozygous genotype HbAHbS have minor problems with anemia
because half their hemoglobin is normal and half is not. Heterozygotes are carriers and they are said to
have sickle-cell trait.
Multiple-Allele Inheritance
An individual inherits only two alleles for each gene, but some genes may have more than two
alternative forms; this is the basis for multiple-allele inheritance. One example of multiple-allele
inheritance is the inheritance of the ABO blood group. The four blood types (phenotypes) of the ABO
group – A, B, AB and O – result from the inheritance of six combinations of three different alleles of a
single gene called the I gene (1) allele IA produces the A antigen, (2) allele IB produces the B antigen,
and (3) allele i produces neither A nor B antigen. Each person inherits two I-gene alleles, one from
each parent, that give rise to the various phenotypes. The six possible genotypes produce four blood
types, as follows:
Genotype
Blood type (phenotype)
IAIAor IAi
A
IBIBor IBi
B
A B
I I
AB
ii
O
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Notice that both IA and IB are inherited as dominant traits, and i is inherited as a recessive trait.
Because an individual with type AB blood has characteristics of both type A and type B red blood cells
expressed in the phenotype, alleles IA and IB are codominant. Both genes are expressed equally in the
heterozygote. Depending on the parental blood types, different offspring may have blood types
different from each other.
Complex Inheritance
Most inherited traits are not controlled by one gene, but instead by the combined effects of two or
more genes, a situation referred to as polygenic inheritance. Complex inheritance is the combined
effects of many genes and environmental factors. In complex inheritance, one genotype can have many
possible phenotypes, depending on the environment, or one phenotype can include many possible
genotypes.
A complex trait shows a continuous gradation of small differences between extremes among
individuals. It is not easy to predict the risk of passing on an undesirable trait when the trait is
complex. Complex traits are difficult to follow in a family because the range of variation is large, the
number of different genes involved usually is not known and the impact of environmental factors may
be incompletely understood.
Autosomes, Sex Chromosomes, and Sex Determination
The 46 human chromosomes in a normal somatic cell can be identified by their size, shape, and
staining pattern to be members of 23 different pairs. In 22 of the pairs, the homologous chromosomes
look alike and have the same appearance in both males and females; the 22 pairs are called autosomes.
Two members of the 23rd pair are termed the sex chromosomes; they look different in males and
females. In females, the pair consists of two chromosomes called X chromosomes. One X
chromosome is present in males, its mate is a much smaller chromosome called a Y chromosome.
When a spermatocyte undergoes meiosis to reduce its chromosome number, it gives rise to two sperm that
contain an X chromosome and two sperm that contain a Y chromosome. Oocytes have no Y chromosomes
produce only X-containing gametes. If the secondary oocyte is fertilized by an X-bearing sperm, the
offspring is female (XX). Fertilization by a Y-bearing sperm produces a male (XY). Thus, an individual’s
sex is determined by the father’s chromosomes.
Both female and male embryos develop identically until about 7 weeks after fertilization. At that
point, one or more genes on the Y chromosome set into motion a cascade of events that leads to the
development of a male. In the absence of normal expression of the gene or genes, the female pattern of
development occurs. The Y chromosome is needed to initiate male development. The prime maledetermining gene is called SRY (sex determining region of the Y chromosome). SRY acts as a
molecular switch to turn on the male pattern of development. Only if the SRY gene is present and
functional in a fertilized ovum will the fetus develop testes and differentiate into a male. In the absence
of SRY gene, the fetus will develop ovaries and differentiate into a female.
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Sex-Linked Inheritance
In addition to determining the sex of the offspring, the sex chromosomes are responsible for the
transmission of several nonsexual traits. Many of the genes for these traits are present on X
chromosome but are absent from Y chromosome. This feature produces a pattern of heredity called
sex-linked inheritance.
Red-Green Color Blindness
One example of sex-linked inheritance is red-green color blindness. This condition is characterized by
a deficiency in either red or green sensitive cones, so red and green are seen as the same color. The
gene for red-green color blindness is recessive (c). Normal color vision, designated C, dominates. The
C/c genes are located only on the X chromosome, so the ability to see colors depends entirely on the X
chromosomes.
Only females who have two Xc genes are red-green color blind. This rare situation can result only
from the mating of a color-blind male and a color-blind or carrier female. Because males do not have a
second X chromosome that could mask the trait, all males with an Xc gene will be red-green color
blind. Traits inherited in the manner described are sex linked traits.
X-Chromosome Inactivation
Because females have two X chromosomes in every cell, females have a double set of all genes on the
X chromosome. A mechanism termed X-chromosome inactivation (lyonization) in effect reduces the
X-chromosome genes to a single set in females. In each cell of a female’s body one X chromosome is
randomly and permanently inactivated early in development, and most of the genes of the inactivated
X chromosome are not expressed. The nuclei of cells in female mammals contain a dark-staining
body, a Barr body, which is not present in the nuclei of cells in males. The Barr body is the inactivated
X chromosome. During inactivation, chemical groups are added to the X chromosome’s DNA that
prevents transcription of RNA. As a result, inactivated X chromosome has a different appearance than
the rest of the DNA. In interphase of non-dividing cells of the female, the inactivated X chromosome
remains tightly coiled and can be seen as a dark-staining body within the nucleus. In a blood smear,
the Barr body of the neutrophils is termed a “drumstick” because it looks like a tiny drumstick-shaped
projection of the nucleus.
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