Heredity and Chromosomes

1. Heredity and Chromosomes

Heredity involves more than the passing of genetic information from our parents. According to evolutionary psychology, our genetic inheritance comes from the most adaptive genes of our ancestors. The gene is considered the basic unit of inheritance. Genes are passed from parents to offspring. They contain the information needed to specify physical and biological traits. Most genes code for specific proteins, or segments of proteins, which have differing functions within the body (National Human Genome Research Institute, 2023).

hint

Adaptive genes are genes that help us adapt to our environment the best way. Our ancestors passed down the 'best genes' because through generations, they have helped us to survive.

Recall the nature versus nurture debate we explored in the first challenge of this unit. Heredity is the ‘nature’ aspect of the debate. It can help us better understand, to some degree, why some of our features such as height, weight, personality, health, etc. may be similar across family members.

To figure out the genetic contributions of this phenomenon, we need to take a step back and ask ourselves, where does this process take place initially? In other words, how and when is our genetic material born - at conception or at birth?

term to know

Heredity

Passing of genetic information from parents to their offspring.

1a. Gametes

There are two types of sex cells or gametes involved in reproduction:

• The male gametes, or sperm
• The female gametes, or ova

The male gametes are produced in the testes through a process called spermatogenesis, which begins at about 12 years of age. The female gametes, which are stored in the ovaries, are present at birth but are immature. Each ovary contains about 250,000 ova but only about 400 of these will become mature eggs (Mackon & Fauser, 2000). Beginning at puberty, one ovum ripens and is released about every 28 days, a process called oogenesis.

After the ovum, or egg, ripens and is released from the ovary, it is drawn into the fallopian tube and reaches the uterus in 3 to 4 days. It is typically fertilized in the fallopian tube and continues its journey to the uterus. At ejaculation, millions of sperm are released into the vagina, but only a few reach the egg and typically, only one fertilizes the egg. Once a single sperm has entered the wall of the egg, the wall becomes hard and prevents other sperm from entering. After the sperm has entered the egg, the tail of the sperm breaks off and the head of the sperm, containing the genetic information from the father, unites with the nucleus of the egg. As a result, a new cell is formed. This cell, containing the combined genetic information from both parents, is referred to as a zygote.

terms to know

Gamete

A male or female reproductive cell.

Spermatogenesis

The production of male gametes in the testes; begins around 12 years old.

Oogenesis

The production or ripening of an ovum and its release every 28 days; begins at puberty.

1b. Chromosomes

While other normal human cells have 46 chromosomes (or 23 pairs), gametes contain 23 chromosomes. A chromosome is a DNA molecule with part or all of the genetic material of an organism. Chromosomes are long threadlike structures found in a cell’s nucleus that contain genetic material known as deoxyribonucleic acid (DNA). DNA is the molecule that carries genetic information for the development and functioning of an organism.

The following illustration of a DNA molecule shows how DNA is made up of two linked strands that wind in a spiral resembling a twisted ladder, forming a helix-like shape. Attached to and joining the strands are a series of nucleotide base pairs: adenine (A), cytosine (C), guanine (G), or thymine (T). The two strands are connected by chemical bonds between the nucleotide base pairs. Notice that the nucleotide base pairs are combinations of either adenine bonded with thymine or cytosine bonded with guanine.

key concept

In each chromosome, sequences of DNA make up genes that control or partially control a number of visible characteristics, known as traits, such as eye color, hair color, and so on. A single gene may have multiple possible variations or alleles. An allele is a specific version of a gene. So, a given gene may code for the trait of hair color, and the different alleles of that gene affect which hair color an individual has.

In a process called meiosis, segments of the chromosomes from each parent form pairs.

did you know

Genetic segments are exchanged as determined by chance. Because of the unpredictability of this exchange, the likelihood of having offspring that are genetically identical (and not twins) is one in trillions (Gould & Keeton, 1997). Genetic variation is important because it allows a species to adapt so that those who are better suited to the environment will survive and reproduce. This is an important factor in natural selection.

terms to know

Trait

A specific characteristic of an individual that can be determined by genes, environmental factors, or a combination of both.

Allele

A specific version of a gene.

Meiosis

A process where parts of the chromosomes from each parent form pairs and genetic segments are exchanged by chance.

1c. Genotypes and Phenotypes

When a sperm and an egg come together, their 23 chromosomes pair up and create a zygote with 23 pairs of chromosomes. Therefore, each parent contributes half the genetic information carried by the offspring. The resulting physical characteristics of the offspring (called the phenotype) are determined by the interaction of genetic material supplied by the parents (called the genotype) as well as environmental factors.

hint

A person’s genotype is the genetic makeup of that individual. Phenotype, on the other hand, refers to the individual’s inherited physical characteristics and observable traits.

As you see your reflection in the mirror you might question what determines whether or not genes are expressed? Maybe you have a gene for a certain eye color but then you end up getting a different eye color. Was it a coincidence? Actually, this is quite complicated. Some features follow the additive pattern, which means that many different genes contribute to a final outcome. Height and skin tone are examples. In other cases, a gene might either be turned on or off depending on several factors, including the gene with which it is paired or the inherited epigenetic markers.

try it

Look in the mirror. Write down the characteristics or things you are seeing reflect back at you. What do you see, your genotype or your phenotype?

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2. Determining the Sex of the Child

Twenty-two of the chromosomes from each parent are similar in length to a corresponding chromosome from the other parent. However, the remaining chromosome, the 23rd pair, looks like an X or a Y.

key concept

Half of the male’s sperm contain a Y chromosome and half contain an X. All of the female’s ova contain X chromosomes. If the child receives the combination of XY, the child will be genetically male. If the child receives the XX combination, the child will be genetically female.

Some potential parents have a clear preference for having a boy or a girl and would like to determine the sex of the child. Through the years, a number of tips have been offered for the potential parents to maximize their chances for having either a son or daughter as they prefer.

EXAMPLE

It has been suggested that sperm which carry a Y chromosome are more fragile than those carrying an X. So, if a couple desires a male child, they can take measures to maximize the chance that the Y sperm reaches the egg. This involves having intercourse 48 hours after ovulation, which helps the Y sperm have a shorter journey to reach the egg; douching to create a more alkaline environment in the vagina because sperm thrive under alkaline conditions; and having the female reach orgasm first so that sperm are not pushed out of the vagina due to the contractions that occur during orgasm.

Today there is new technology available that makes it possible to isolate sperm containing either an X or a Y, depending on the preference. That sperm can be used to fertilize a mother’s egg. Preimplantation genetic diagnosis (PGD) can be used to select only embryos of the desired sex to be implanted during in-vitro fertilization (IVF). IVF is the joining of the egg and sperm outside of the body, in a lab petri dish. After the egg is fertilized in the laboratory, it is implanted in a woman’s uterus or frozen for future use. PGD is controversial and fertility centers and medical organizations discourage it if there is no real medical reason to select the sex of the child.

term to know

Preimplantation Genetic Diagnosis (PGD)

A procedure where selected embryos of the desired sex are implanted during in-vitro fertilization.

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3. Genetic Variation & Inheritance

Genetic variation, the genetic difference between individuals, is what contributes to a species’ adaptation to its environment. In humans, genetic variation begins with an egg, several million sperm, and fertilization. The egg and sperm each contain 23 chromosomes, which make up our genes. A single gene may have multiple possible variations or alleles (a specific version of a gene), resulting in a variety of combinations of inherited traits.

Genetic inheritance of traits for humans is based upon Gregor Mendel’s model of inheritance. For genes on an autosome (any chromosome other than a sex chromosome), the alleles and their associated traits are autosomal dominant or autosomal recessive. In this model, some genes are considered dominant because they will be expressed. Others, termed recessive, are only expressed in the absence of a dominant gene. Some characteristics which were once thought of as dominant-recessive, such as eye color, are now believed to be a result of the interaction between several genes (McKusick, 1998). Dominant traits include curly hair, facial dimples, normal vision, and dark hair. Recessive characteristics include red hair, pattern baldness, and nearsightedness.

Sickle cell anemia is an autosomal recessive disease; Huntington disease is an autosomal dominant disease. Other traits are a result of partial dominance or codominance in which both genes are influential.

EXAMPLE

If a person inherits both recessive genes for cystic fibrosis, the disease will occur. But if a person has only one recessive gene for the disease, the person would be a carrier of the disease but would not have symptoms of the disease itself.

In the example depicted in the table below, the normal (dominant) gene is referred to as “N,” and the recessive gene for cystic fibrosis is referred to as “c.” The normal gene is dominant, which means that having the dominant allele either from one parent (Nc) or both parents (NN) will always result in the phenotype associated with the dominant allele.

• When someone has two copies of the same allele, they are said to be homozygous for that allele.
• When someone has a combination of alleles for a given gene, they are said to be heterozygous.

For example, cystic fibrosis is a recessive disease which means that an individual will only have the disease if they are homozygous for that recessive allele (cc). Imagine that a woman who is a carrier of the cystic fibrosis gene has a child with a man who also is a carrier of the same disease. What are the odds that their child would inherit the disease? Think about these facts:

• Both the woman and the man are heterozygous for this gene (Nc).
• We can expect the offspring to have a 25% chance of having cystic fibrosis (cc), a 50% chance of being a carrier of the disease (Nc), and a 25% chance of receiving two normal copies of the gene (NN).

terms to know

Genetic Variation

The genetic difference between individuals.

Mendel’s Model of Inheritance

A model that helps explain how human traits are inherited.

Autosome

Any chromosome except for a sex chromosome.

Autosomal Dominant

A gene that is expressed.

Autosomal Recessive

A gene that is expressed only in the absence of the dominant gene.

Homozygous

Having two copies of the same allele for a given gene.

Heterozygous

A combination of alleles for a given gene.

3a. Gene Mutations

Where do harmful genes that contribute to diseases like cystic fibrosis come from? Gene mutations provide one source of harmful genes. A mutation is a sudden, permanent change in a gene. Many mutations can be harmful or lethal. Once in a while a mutation benefits a person by giving that person an advantage over those who do not have the mutation.

Recall the theory of evolution. It asserts that individuals best adapted to their particular environments are more likely to reproduce and pass on their genes to future generations. In order for this process to occur, there must be competition - more technically, there must be variability in genes (and resultant traits). This allows for variation in adaptability to the environment.

big idea

If a population consisted of identical individuals, then any dramatic changes in the environment would affect everyone in the same way. There would be no variation in selection. In contrast, diversity in genes and associated traits allows some individuals to perform slightly better than others when faced with environmental change. This creates a distinct advantage for individuals best suited for their environments in terms of successful reproduction and genetic transmission.

term to know

Mutation

A sudden, permanent change in a gene.

REFERENCES

Gould, J. L. (1997). Biological science. New York: Norton.

Green, E. (2023, January 20). Gene. National Human Genome Research Institute. Retrieved January 24, 2023, from www.genome.gov/genetics-glossary/Gene.

Mackon, N., & Fauser, B. (2000). Aspects of ovarian follicle development throughout life. Hormone Research, 52, 161-170.

McKusick, V. A. (1998). Mendelian inheritance in man: A catalog of human genes and genetic disorders. Baltimore, MD: Johns Hopkins University Press.