Chapter 14 – The Human Genome Section 14-1: Human Heredity

Humans have 23 pairs of chromosomes, or 46 chromosomes total, in a diploid cell Karyotype = a picture of the chromosomes arranged in homologous pairs

2 of the 46 chromosomes are known as sex chromosomes  they determine and individuals sex Females have 2 X chromosomes Males have 1 X and 1 Y chromosome

Males and females are born in almost a 50:50 ratio because of the way sex cells segregate during meiosis The other 44 chromosome are known as autosomes

Human genes are inherited according to the Mendelian principles To apply Mendelian genetics, scientists have to identify a trait inherited by a single gene  they must also establish that the trait is inherited and not the result of environmental influences

A pedigree chart shows the relationships within a family and can help determine if the gene is inherited Genetic counselors analyze pedigree charts to infer the genotypes in family members

If a trait is dominant and an individual shows the recessive phenotype, they must be homozygous recessive This also implies that the person who passed the trait on was heterozygous because they were able to pass along a recessive allele

An individual’s phenotype is only partially determined by the genotype Many traits are strongly influenced by environmental factors including nutrition and exercise These environmental effects are not inherited; genes are Genes may be denied a proper environment in which to reach full expression in one generation, but given the right environment can be seen more in later generations

The human genome is our complete set of genetic information and includes tens and thousands of genes It took scientists lots of time to identify genes that directly control a single human trait Some of the first human genes identified were those that control blood type

Human blood has been broken up into different groups – your blood group is very important because if you get a transfusion from the wrong blood group it can be fatal The best known controls of blood type are the ABO group and the Rh group

The Rh group is controlled by one gene with 2 alleles; positive and negative Positive is dominant

The ABO group has 3 alleles for the gene; IA, IB, and i IA and IB are codominant, a person with both alleles is blood type AB i is recessive, so a person with two i alleles makes no antigens and is blood type O

IA IA or IAi would produce blood type A IB IB or IBi would produce blood type B If a medical worker tells you your blood type, they might say A+, they are referring to both groups of genes

Many of the human genes that became known were associated with a genetic disorder Several examples of genetic disorders caused by recessive alleles are albinism, cystic fibrosis, and Tay-Sachs disease Several examples of genetic disorder caused by dominant alleles are achondroplasia and Huntington’s disease

Sickle cell is a disorder associated with codominant alleles Several of these disorders are caused by just a small change in DNA that ultimately affects the structure of a protein  these proteins are so important that when changed they can have lethal effects

Section 14-2: Human Chromosomes

A human diploid cell contains 46 chromosomes Chromosomes 21 and 22 are the smallest human autosomes  they were the first two chromosomes to be sequenced and their structural features are representative of other chromosomes On both chromosomes there are long stretches of DNA that do not code for genes, these long stretches represent unstable sites where mutations can occur and cause problems

Genes that are close enough together on a chromosome tend to be inherited together Genes that are located on the X and Y chromosomes are sex-linked genes More than 100 sex-linked genetic disorders have been mapped to the X chromosome; the Y chromosome is smaller by comparison and only contains a few genes

Males have just one X chromosome, so all X alleles are expressed in males, even if they are recessive There are 3 genes associated with color vision on the X chromosome  in males if there is a defect in any of the genes it produces colorblindness

Hemophilia is another sex-linked disorder  a protein for blood clotting is missing and a person with hemophilia can bleed to death from a minor cut Muscular dystrophy is also a sex-linked disorder  it causes a progressive weakening and loss of muscle tissue

The most common error during meiosis is when homologous chromosomes fail to separate (nondisjunction) If nondisjunction occurs, abnormal numbers of chromosomes may find their way into gametes and a disorder of chromosome numbers may result

Trisomy = when an individual ends up with 3 copies of a chromosome The most common form of trisomy is Down syndrome  it produces mild to severe mental retardation and other birth defects

Nondisjunction can occur with the X and Y chromosomes as well In Turner’s syndrome, a female is only born with one X chromosome  as a result her sex organs do not develop at puberty and she is sterile In Klinefelter’s syndrome, males end up with an extra X chromosome There has not been a case where a baby was born without an X chromosome, showing that its genes are essential for survival

Human Molecular Genetics Section 14-3: Human Molecular Genetics

Knowing more about the sequence of human genes has been useful in several ways Parents can test for recessive alleles that may cause genetic disorders for their children If parents are worried their child may have Tay-Sachs disease, they can have the fetus tested and make decisions based on the findings

DNA fingerprinting takes advantage of the fact that individuals each have their own DNA sequence DNA fingerprinting does not analyze important genes (those are usually identical among the population), but instead analyzes sections of DNA that have little or no function and vary widely DNA fingerprinting uses restriction enzymes and gel electrophoresis to analyze and compare DNA

The human genome project was started in 1990 and completed in 2003 Some of the major goals of the project were to identify approximately 20,000 to 25,000 genes in human DNA and help to improve data analysis of DNA With the information from the project, some scientists hope to be able to cure genetic disorders through gene therapy

In gene therapy, an absent or faulty gene is replaced by a normal, working gene Gene therapy is still a high-risk experimental procedure