Genetic Traits, Phenotypes, and Heritability: Key Concepts in Human Genetics

What is a genetic trait?

Characteristics or attributes of an organism expressed by genes, including physical and behavioral traits.

What is a phenotype?

An observable physical or biochemical property of an organism that indicates the state of a trait.

Give an example of a trait and its phenotypes.

Trait: Eye color; Phenotypes: Blue, brown, hazel.

What are Mendelian traits?

Traits controlled by single gene pairs, exhibiting discontinuous variation.

What is the difference between discontinuous and continuous variation?

Discontinuous variation has distinct classes (e.g., tall or short), while continuous variation shows a range of phenotypes (e.g., height).

What are polygenic traits?

Traits determined by two or more genes, which may or may not be significantly influenced by the environment.

Define multifactorial traits.

Polygenic traits resulting from the effects of two or more genes and one or more environmental factors.

What is the additive model of complex traits?

Assumes each gene has an equal effect on phenotype, with small contributions from each gene.

How does eye color illustrate polygenic inheritance?

Eye color is controlled mainly by two genes, each with dominant and recessive alleles, leading to multiple phenotypic classes.

What influences human skin color?

Human skin color is controlled by 3 to 4 genes and is influenced by environmental factors such as latitude.

What is heritability in the context of traits?

A measurement that estimates the proportion of phenotypic variation of a trait due to genetic differences.

What does a heritability value of 1.0 indicate?

Genetic factors contribute 100% to phenotypic variation, with little to no effect from environmental factors.

What does a heritability value of 0.0 indicate?

Little to no genetic contribution to phenotypic variation, with environmental factors contributing 100%.

What is dermatoglyphics?

The study of fingerprints, which are influenced by both genetic and environmental factors.

How are fingerprints an example of multifactorial traits?

Fingerprints are controlled by multiple genes and environmental factors, resulting in a wide range of phenotypes.

What is the significance of studying phenotypic variation in populations?

Phenotypic variations are best analyzed in populations rather than individuals to understand complex traits.

What is the role of environmental variance in phenotypic variation?

Environmental variance accounts for differences in traits attributed to environmental factors.

What is genetic variance?

Phenotypic variance of a trait in a population attributed to genotypic differences.

What are examples of complex traits?

Examples include fingerprint patterns, intelligence, cancer, autism, and mental illness.

What is the purpose of twin studies in genetics?

To separate and evaluate the genetic and environmental factors responsible for phenotypic variation.

What are monozygotic (MZ) twins?

Genetically identical twins derived from a single fertilization involving one egg and one sperm.

What are dizygotic (DZ) twins?

Twins derived from two separate fertilization events, each involving one egg and one sperm, sharing about 50% of their genes.

How does the correlation coefficient relate to heritability?

It indicates the degree of genetic relatedness between two individuals, used to calculate heritability values.

What is the correlation coefficient for identical twins?

1.0, indicating they share all their genes.

What does a higher concordance rate in MZ twins suggest?

A greater heritability for the trait being studied.

Define concordance in the context of twin studies.

The agreement between traits exhibited by both twins, where concordant means both have the trait and discordant means only one has it.

What is the estimated heritability component of obesity based on twin studies?

About 70%.

What is the role of the ob gene in obesity?

It encodes the weight-controlling hormone leptin, which regulates satiety and energy consumption.

What are multifactorial traits?

Traits that are polygenic and result from interactions of two or more genes and one or more environmental factors.

What is the significance of the concordance rates found in twin studies of homosexual orientation?

MZ twins had a concordance rate of 65.8%, while DZ twins had a rate of 30.4%, suggesting a biological basis for sexual orientation.

What does GWAS stand for and what is its purpose?

Genome-Wide Association Studies; it determines if specific DNA pieces are significantly associated with particular traits.

What were the findings of the 2019 GWAS related to same-sex sexual behavior?

Five autosomal loci were significantly associated with same-sex sexual behavior, indicating multiple genetic influences.

What is the genetic correlation between autism spectrum disorder (ASD) and other complex disorders?

Strong genetic correlations exist between ASD and disorders like schizophrenia, major depression, and educational attainment.

What is the estimated heritability for anorexia nervosa based on twin studies?

55% for MZ twins and 7% for DZ twins.

What is the concordance rate for autism in MZ twins?

90%.

What is the concordance rate for schizophrenia in DZ twins?

10%.

How does the environment affect the heritability of traits in twin studies?

DZ twins share the same womb environment but have different genotypes, allowing for the evaluation of environmental impacts.

What is the significance of the findings from the twin studies on gender identity?

They suggest that gender identity may be a multifactorial complex trait with a heritable polygenic component.

What is the concordance rate for bipolar disorder in MZ twins?

33%-80%.

What is the concordance rate for hypertension in DZ twins?

48%.

What is the role of leptin in fat storage?

Leptin regulates the conversion of food energy into fat and the rate of energy consumption.

What is the estimated heritability for cleft lip with or without cleft palate in MZ twins?

40%.

What does the term 'polygenic' refer to in genetics?

Traits that are influenced by multiple genes.

What is a karyotype?

A karyotype is a picture of an organism's chromosomes, used to examine their number and appearance for abnormalities.

How many chromosomes do humans have?

Humans have 23 pairs, totaling 46 chromosomes.

What does diploid (2n) mean?

Diploid refers to the full set of chromosomes in an organism, which is 46 in humans.

What does haploid (n) refer to?

Haploid refers to the number of chromosomes in gametes, which is 23 in humans.

What are autosomes?

Autosomes are chromosomes other than the sex chromosomes; in humans, chromosomes 1 to 22 are autosomes.

What are sex chromosomes in humans?

The sex chromosomes in humans are X and Y, which are involved in sex determination.

What is polyploidy?

Polyploidy is a condition where an organism has more than two complete sets of chromosomes.

What is aneuploidy?

Aneuploidy is a condition where there is an abnormal number of chromosomes, either more or fewer than the normal set.

What is the significance of centromere location in chromosome identification?

Centromere location helps classify chromosomes as metacentric, submetacentric, or acrocentric, which contributes to their characteristic shapes.

What are the three types of centromere locations?

1. Metacentric: centrally placed centromere; 2. Submetacentric: centromere closer to one end; 3. Acrocentric: centromere near the end.

What is the purpose of chromosome banding?

Chromosome banding is used to produce specific patterns that help identify homologous chromosomes and detect structural abnormalities.

What does the 'p' arm of a chromosome represent?

The 'p' arm represents the short arm of a chromosome.

What does the 'q' arm of a chromosome represent?

The 'q' arm represents the long arm of a chromosome.

What is G-banding?

G-banding is a technique that uses trypsin and Giemsa stain to produce a banding pattern on chromosomes for identification.

What is the significance of the discovery made in 1959 regarding Down syndrome?

It was discovered that Down syndrome is caused by an extra copy of chromosome 21.

What are common methods for obtaining cells for chromosome studies?

Common methods include using lymphocytes, skin cells, and invasive procedures like amniocentesis and chorionic villus sampling (CVS).

What is the process of making a karyotype?

The process involves stimulating cell division, stopping mitosis at metaphase, staining the chromosomes, and arranging them for analysis.

What is the role of phytohemagglutinin in karyotype preparation?

Phytohemagglutinin is added to stimulate mitosis in the cells for karyotype analysis.

What does C-banding reveal?

C-banding reveals the centromeric region of chromosomes by chemically treating metaphase spreads to extract DNA from the arms.

How is a karyotype designated for a normal human female?

A normal human female karyotype is designated as 46, XX.

How is a karyotype designated for a normal human male?

A normal human male karyotype is designated as 46, XY.

What is the impact of Rosa's Law in relation to intellectual disabilities?

Rosa's Law removed the terms 'mental retardation' and 'mentally retarded' from federal policies, replacing them with 'individual with an intellectual disability.'

What are the four common chromosome staining procedures?

1. G-banding; 2. Q-banding; 3. R-banding; 4. C-banding.

What is the aim of constructing and analyzing karyotypes?

The aim is to identify chromosome abnormalities and variations in chromosomal number and structure.

What does the term 'chromosomal aberration' refer to?

Chromosomal aberration refers to any change in the normal structure or number of chromosomes that can affect health.

What is a karyotype designated as for Down Syndrome?

47, XY, +21, also known as Trisomy 21.

What types of cells can be used to make a karyotype?

Lymphocytes, skin cells, cells from biopsies, and tumor cells.

What are two invasive procedures for sampling cells before birth?

Amniocentesis and Chorionic Villus Sampling (CVS).

What is the purpose of amniocentesis?

To collect fluid surrounding the developing fetus for diagnosing genetic and developmental disorders.

What is the risk percentage of miscarriage due to amniocentesis?

0.2% - 0.3% chance of miscarriage.

What is the advantage of Chorionic Villus Sampling (CVS) over amniocentesis?

CVS can be performed earlier (8-10 weeks) and has faster karyotype preparation time.

What does cff DNA stand for?

Cell-free fetal DNA.

What percentage of non-cellular DNA in a mother's blood does cff DNA represent?

11-13%.

What is the normal diploid number of chromosomes in humans?

46 chromosomes (2n).

What is triploidy and its karyotype designation?

Triploidy is a chromosomal number of 69 (3n), with three copies of all autosomes and sex chromosomes.

What is the most common form of polyploidy?

Triploidy, which accounts for 15-18% of all miscarriages.

What is monosomy?

A condition where one member of a chromosomal pair is missing, resulting in (2n - 1).

What is trisomy?

A condition where one chromosome is present in three copies, resulting in (2n + 1).

What are the karyotype designations for Trisomy 13 and Trisomy 18?

Trisomy 13: 47, +13; Trisomy 18: 47, +18.

What is the leading cause of aneuploidy?

Nondisjunction during meiosis.

What happens during nondisjunction in Meiosis I?

All gametes will be abnormal, two with extra chromosomes, two with missing

What happens during nondisjunction in Meiosis II?

Produces two normal haploid gametes and two abnormal gametes, one with an extra chromosome (trisomy) and one missing a chromosome (monosomy).

What percentage of live births have a chromosomal abnormality?

1 in 170 live births.

What is the significance of the 800 chromosomally abnormal embryos?

Of 10,000 early embryos, about 800 have chromosome abnormalities, leading to a high rate of miscarriage.

What is the karyotype designation for a triploid individual?

69, XXX, 69, XXY, or 69, XYY.

What is the chromosomal composition of a tetraploid human?

92 total chromosomes (4n).

What are the common abnormalities associated with triploidy?

Fused fingers and toes, enlarged head, and other malformations.

What is the typical outcome for triploid pregnancies?

Over 99% result in miscarriage, with 1 in 10,000 live births being triploid.

What occurs during nondisjunction in meiosis I?

Failure of homologous chromosomes to separate

What are the outcomes of nondisjunction in meiosis II?

Two normal and two abnormal gametes, with one abnormal gamete having an extra chromosome (trisomy) and the other missing a chromosome (monosomy).

What is autosomal monosomy and its outcome?

A lethal condition that is eliminated early in development, often resulting in spontaneous abortion.

Which autosomal trisomies can result in live births?

Trisomy 13, trisomy 18, and trisomy 21.

What is Trisomy 21 also known as?

Down syndrome.

What are the common characteristics of Down syndrome?

Physical growth, behavior, and mental development are affected; 40% have congenital heart defects; increased susceptibility to respiratory infections, leukemia, and Alzheimer's.

What is the estimated U.S. population of individuals with Down syndrome?

Over 400,000.

What is the average lifespan of a person with Down syndrome today?

Approximately 60 years.

What is Turner syndrome?

A condition caused by monosomy of the X chromosome, occurring in 1 in 10,000 live births, characterized by physical differences in height and sexual development, but no intellectual disability.