Genetics and Heredity

Genetics and Heredity Lab Exercise 40 BI 233

Genome • All of the genes in all of the chromosomes together are called the genome • The study of all the genes in the human species is called genomics

Definitions • Heredity = the inheritance of traits • Genetics = the study of mechanisms of heredity. • Genes: A portion of a DNA strand that functions as a hereditary unit, is located at a particular site on a specific chromosome, and codes for a specific protein or polypeptide

Gene Expression • A person has two genes for every inherited characteristic. • If one gene is always expressed, geneticists call that gene dominant. • A gene that is not expressed when its mate is different is termed a recessive gene

Gene expression • Codominanceis the condition in which an individual heterozygous for a gene exhibits the phenotypes of both homozygote's (example AB blood) • Incomplete dominance is when neither of two alleles is completely dominant.(example: sickle cell anemia at organism level)

Gene Expression • Polygenic inheritance occurs when two or more genes whose additive effects determine a particular trait.(skin color) • Multi allelic inheritance: refers to different versions of one trait on the same gene (Blood types)

Inherited characteristics • The characteristic actually expressed in an individual is called a phenotype. • The genetic constitution of an individual, along with environmental influences is called the genotype. • If both genes for a trait are the same the individual is homozygous. • If genes are not the same they are heterozygous.

Genes and Chromosomes • The position of a gene on a chromosome is called a locus. • Genetic maps show the relative locations of genes along the chromosome. • There can be alternate versions of genes called alleles which produce different phenotypes

Centromere • Each chromosome often has a constriction along its length called a centromere. • The shorter arm is placed on top in a karyotype and is called the p arm, the longer arm is on the bottom and is called the q arm. • Numbered regions and subregions are then assigned from the centromere outward • The breast cancer gene BRCA1 is located at 17q21 meaning that it is on the long arm of chromosome 17 in the region of 21 • Used for gene mapping

Chromosomes • We inherit 23 DNA molecules from our mothers and 23 DNA molecules from our fathers for a total of 46. • 22 pairs of these chromosomes are called autosomes • The remaining pair are called sex chromosomes.

Taste differences • Sodium benzoate test – The ability to taste something sweet, salty, or bitter in the paper is dominant. • PTC (phenylthiocarbamide) test The ability to sense a bitter taste is dominant. • Thiourea test – the ability to taste something bitter is dominant

Anatomical characteristics of hand • Bent little finger – If distal phalanx of the little finger bends toward the fourth finger, you have dominant trait. • Middigital hair – hair on middle phalanges is dominant. • Hitchhiker’s thumb- If you can hyperextend the distal joint of the thumb noticeable, you have the recessive trait.

Facial Features • Pigmented anterior of the iris – If you have pigment on the anterior and posterior of the iris, your eyes are green, brown, black and hazel. • If you lack pigment on the anterior aspect of the iris, yours eyes are blue or grey. • Pigmentation is dominant

Phenotypes of facial features • Attached earlobes – Lobe of the ear is attached rather than free, you have the recessive trait. • Widow’s peak – hairline straight across forehead is recessive trait • Tongue roll – curling tongue is dominant • Freckles – if your face is free of freckles you have the recessive form of this characteristic

ABO blood types • There are two dominant genes for ABO blood types • Type A (IA):signifies the A antigen • Type B (IB): signifies the B antigen. • Type O (i): is recessive to both Type A and Type B • A person with Type A blood can have IAIAor IAi • Type B blood can have IBIBor IBi

Rh Blood Type • The presence of the Rh antigen is dominant

Karyotypes • Standard chromosome chart • Major clinical genetic tool • Chromosomes are presented by size and physical landmarks that appear during mitotic metaphase when DNA is coiled tightly • Karyotypes can be used to diagnose many genetic conditions as well as reveal effects of environmental toxins.

Non-disjunction problems • When the homologous chromosomes or sister chromatids fail to separate at anaphase • An extra chromosome is called trisomy and a missing chromosome is called monosomy • Trisomy 21 (Down’s syndrome) • Signs include impaired physical development; short stature; relatively flat face with a flat nasal bridge and mental retardation • Trisomy 13 & 18 are other chromosomal trisomies but fetuses do not usually survive until birth.

Turner’s Syndrome • XO • Have only one sex chromosome • Designated as 45,X indicating they have 45 chromosomes and one X chromosome. • Not noticeable until puberty when they fail to develop secondary sex characteristics and are usually infertile. • Webbed neck • About 97% of fetuses with Turner syndrome die before birth.

Klinefelter’s Syndrome • If an XX egg is fertilized by a Y-bearing sperm the result is an XXY combination. • Sterile males with usually average intelligence with undeveloped testes, sparse body hair, usually long arms and legs and enlarged breasts. • Often goes undetected until puberty.

Chromosome Duplications • Fragile X syndrome • Caused by repeats on the X chromosomes • Most people have about 29 repeats but persons with Fragile X have over 700 repeats due to duplication. • A major cause of mental retardation and is found in more males because of the single X chromosomes

Translocation defect • Different chromosomes exchange or combine parts • Exposure to certain viruses, drugs and radiation can cause translocations, but they often arise for no apparent reason. • No gain or loss of genetic material is involved • Can be a cause of Down’s syndrome when a Robertson translocation produces 3 copies of the long arm of chromosome 21

Pedigree • The study of a family tree and the careful compilation of phenotypic records of both families over a number of generations. • By studying human families for affected and unaffected members over several generations we can usually tell whether a trait is autosomal recessive or is carried on a sex chromosome.

Autosomal Recessive Traits • Characteristics: • Most affected individuals have unaffected parents • All children of two affected individuals are affected • The risk of an affected child from a mating of two heterozygotes is 25% • Expressed in both males and females • Examples: albinism, cystic fibrosis, PKU, etc.

Autosomal Dominant • Characteristics: • Every affected individual should have at least one affected parent • Affects males and females equally • Two affected individuals may have an unaffected child because most affected individuals are heterozygous. • Examples: Huntington disease, Marfan syndrome

X-linked recessive • Characteristics: • Phenotypic expression is much more common in males than females • Affect males get the mutant allele from their mothers and transmit it to all their daughters but not to any sons. • Examples: colorblindness, hemophilia, muscular dystrophy (some forms)

Hemophilia in Royal Family Is this trait autosomal recessive, autosomal dominant or sex linked? How can you tell?

Probability • Is the likelihood of a the offspring of a particular set of parents will have a certain inherited condition. • Genetic counselors work with prospective parents to determine their possible genotypes for a variety of traits and then predict the probability of their children having those traits.

Activities • Follow the instructions in your lab manual to determine your phenotype for the listed traits. • After you have determined your phenotype then determine your possible genotypes. • Answer the questions in your lab manual using the Punnettsquares and pedigrees • Be able to answer questions concerning both karyotypes and pedigrees

Genetics and Heredity