Chapter 12 Notes – Human GeneticsLeslie Reyesp.7

12.1 – The Nucleus Contains an information-rich genome.

1. Describe how DNA is packed within the nucleus.2. Explain the significance of the Human Genome Project.

DNA Packing in a Single Cell Each chromosome consists of a single DNA molecule. The complete set of genetic material in an organism, as defined by the

order of bases in the DNA is called it’s genome. First, DNA wraps around small proteins called histones. Next, DNA the DNA is wrapped into a tight helical fiber, and then coils

further into a thick “supercoil”. Looping and folding further compacts the DNA in each chromosome.

12.2 – Accidents affecting chromosomes can cause disorders.

1. Relate down Syndrome and Nonseperation of chromosomes2. Describe how Chromosomes can be damaged. 3. Explain how a “jumping gene” can effect other genes.

Down Syndrome Trisomy 21 results from an error during either stage of meiosis, but most

commonly during meiosis I. Many embryos with trisomy chromosome 21 do survive. People with trisomy 21 have a general set of symptoms called Down

Syndrome. These symptoms include heart defects an impaired immune system, and

carrying degrees of mental disability.

Nonseperation of Chromosomes Trisomy 21 and other errors in chromosome number are usually caused

by homologous chromosomes or sister Chromatids failing to separate during meiosis, and event called nondisjunction.

It can occur in anaphase of meiosis I or II, resulting in gametes with abnormal number of chromosomes.

As a woman gets older, she is more likely to have offspring with trisomy 21.

Damaged Chromosomes Even if all the chromosomes are present in normal numbers in a cell,

changes in a chromosome structure may also cause disorders. The first type of change, a duplication, occurs when part of a

chromosome is repeated. Duplications within certain chromosomes are not always fatal but often

result in developmental abnormalities. A second type of change, a Deletion, occurs when a fragment of a

chromosome is lost. Large deletions have a very serious effect on the body. Another type of change, an Inversion, involved reversing a fragment of

the original chromosome. Because most of the genes are still present in their normal number,

inversions are less likely than deletions or duplications to produce harmful effects.

A Translocation occurs when a fragment of one chromosome attaches to a nonhomologous chromosome.

Sometimes a translocation results in two different chromosomes exchanging parts.

Jumping Genes Another type of change in chromosomes involves single genes that can

move around. Mcclintock discovered that these “jumping genes” could land in the middle

of other genes and disrupt them. McClintock’s jumping genes are now called transposons.

12.3 – Mendel’s principles apply to Humans.

1. Summarize the information provided in a pedigree.2. Explain how recessive, dominant, and sex-linked disorders are inherited.3. Describe how it is possible to predict certain genetic disorders.

Working with Human Pedigrees A Pedigree is a family tree that records and traces the occurrence of a

trait in a family. A typical pedigree uses squares to represent males and circles to

represent females. The colored shapes represent individuals that show the trait. Parents are connected buy horizontal lines, with their children beneath

them.

Disorders Inherited as Recessive Traits Over a thousand human genetic disorders are known to have Mendelian

inheritance patterns. Most human genetic disorders are recessive.

They also range in severity. Some such as albinism are not life-threatening.

Others such as Tay-Sachs disease which leads to major nerve damages, are fatal.

The vast majority of people with recessive disorders are born to parents who are heterozygous and do not have the recessive disorder.

An individual who has one copy of the allele for a recessive disorder and does not exhibit symptoms is a carrier or the disorder.

The recessive allele for a very serious disorder, cystic fibrosis, is carried by 1 in every 25 people of European ancestry.

The gene involved codes for a protein that functions in the lungs and other organs.

A mutation in the gene makes the protein defective; this results in certain organs secreting excessive amounts of very thick mucus.

This mucus can interfere with breathing, digestion and liver function.

Disorders inherited as Dominant Traits. A smaller number of human disorders are inherited as dominant traits. One Ex.; being born with extra fingers and toes. A very serious from of dwarfism called achondroplasia causes the persons

torso to develop properly, but the arms and legs are short. All individuals with this disorder are heterozygous – They have a single

copy of the dominant allele. Inheriting two copies is fatal. Dominant alleles that are lethal are, in fact, much less common than lethal

recessives. This is because for most dominant disorders, the affected person dies

before producing any offspring that could inherit the allele. In contrast, lethal recessive alleles usually do not have a large effect on

the heterozygous carriers. As a result, alleles for recessive disorders may pass undetected from

generation to generation, until two carriers happen to produce a homozygous offspring with the disorder.

Lethal dominant disorders that don’t cause death until adulthood can affect multiple generations of a family.

One example is Huntington’s disease; which causes a degeneration of the nervous system that usually does not begin until middle age.

As it progresses, it causes loss of mental ability and muscle control, and eventually death.

Sex-Linked Disorders Red-Green colorblindness is a common sex-linked disorder that involves a

malfunction of light-sensitive cells in the eyes. It’s rare – but not impossible – for females to exhibit sex-linked (X-Linked)

traits.

Predicting and Treating Genetic Disorders A genetic counselor is trained to collect and analyze data about

inheritance patterns and to explain the results and their significance.

12.4 – Genetic changes contribute to Cancer.

1. explain how mutations to genes that play a role in regulating the cell cycle can lead to cancer.

2. Describe how inheriting certain mutations can increase a person’s risk for cancer.

Cancer Genes One class of genes produces proteins called growth factors that initiate

cell division. The other class of genes, known as tumor-suppressor genes, produces

proteins that stop cell division in particular situations. A single mutation to a single gene involved in regulating the cell cycle

does not usually lead to cancer. Rather, cancer develops when several mutations to such genes

accumulate. For example, the path to cancer often starts with a mutation to a gene that

produces growth factors. If the mutation results in producing too much growth-factor activity, the

gene may have become an oncogene – a cancer causing gene.

“Inherited” Cancer Most mutations that lead to cancer occur in the organ where the cancer

starts. Sometimes, a mutation to one of more of these same genes does not

occur in a cell that gives rise to gametes (preventing inheritance).