chapter 29 development

Meiosis

Cell division that involves DNA replication followed by two cell divisions; occurs only in the gonads and produces haploid gametes; ensures the correct chromosome number in the zygote; occurs through the same process in males and females, but with different outcomes; haploid sperm (n = 23) + haploid oocyte (n = 23) = diploid zygote (2n = 46).

Gonads

Mixed glands that produce sex hormones and gametes; male gonads are the testes and female gonads are the ovaries.

Gametes

Haploid cells produced by meiosis; the male gamete is the spermatozoon and the female gamete is the ovum.

Somatic Cells

Diploid cells produced by mitosis; responsible for growth, repair, and regeneration of tissues; all body cells except gametes.

Gametogenesis

The processes that result in a fully functional gamete; male gametogenesis is spermatogenesis and female gametogenesis is oogenesis.

Diploid (2n) vs. Haploid (n)

Diploid cells contain the normal chromosome number of 46 and are found in somatic cells; haploid cells contain 23 chromosomes and are found in gametes; sperm (n) + ovum (n) = zygote (2n).

Homologous Chromosomes

Humans have 23 pairs of homologous chromosomes; each chromosome in a pair codes for the same genes; one chromosome comes from the father and the other from the mother; autosomes are chromosomes 1-22 and code for everything except genetic sex; sex chromosomes are pair 23 and code for genetic sex and other traits.

Gene

Locus, and Alleles, A gene is a segment of DNA that codes for a trait; a locus is the location of a gene on a chromosome; alleles are different forms of a gene at a specific locus.

Homozygous vs. Heterozygous

Homozygous means the alleles for a specific trait are the same on both homologous chromosomes; heterozygous means the alleles are different on each homologous chromosome.

Dominant Allele

An allele that masks or suppresses the expression of a recessive allele when present; determines the phenotype; represented by a capital letter.

Recessive Allele

An allele whose expression is masked when a dominant allele is present; represented by the same letter as the dominant allele but in lowercase.

Genotype vs. Phenotype

Genotype is a person's genetic makeup for a specific trait; phenotype is the physical expression of that trait.

Heterozygous (Jj)

Has one dominant and one recessive allele; expresses the dominant phenotype; example: double-jointed thumbs.

Homozygous Dominant (JJ)

Has two copies of the dominant allele and expresses the dominant phenotype; example: double-jointed thumbs.

Homozygous Recessive (jj)

Has two copies of the recessive allele and expresses the recessive phenotype; example: straight thumbs.

Mendel's Principles of Inheritance / Chromosomal Theory of Inheritance

Principles that explain how inheritance occurs; four major processes promote genetic variability: segregation of chromosomes, independent assortment of chromosomes, crossing over of homologous chromosomes, and random fertilization of eggs by sperm.

Genetic Variability

Differences in genetic combinations among individuals; necessary for the overall health and survival of a species.

Segregation

Occurs during anaphase I of meiosis when the two alleles for one trait separate and are distributed into different daughter cells; for example, a heterozygous person (Jj) produces cells receiving either J or j.

Nondisjunction

An error in which homologous chromosome pairs fail to separate correctly during meiosis I or II; results in an abnormal number of chromosomes called aneuploidy in the resulting egg or sperm; the zygote may also have an abnormal chromosome number, potentially leading to cancer, infertility, or various syndromes; about 80% of first-trimester miscarriages are due to aneuploidy.

Independent Assortment

Alleles for two different traits on two different chromosomes are distributed independently of one another; for example, a person with Hh and Jj can produce gametes HJ, Hj, hJ, and hj; inheriting H or h is independent of inheriting J or j; independent assortment alone produces about 8.5 million possible combinations.

Punnett Square

A grid used to predict possible genotypes and phenotypes of potential offspring.

Monohybrid Cross

A Punnett square that shows the genetic combinations for one trait.

Punnett Square: Two Heterozygous Parents (Aa × Aa)

Produces 25% homozygous dominant (AA), 50% heterozygous (Aa), and 25% homozygous recessive (aa).

How to Complete a Punnett Square

Identify the gene and its dominant and recessive alleles; assign a capital letter to the dominant allele and the same lowercase letter to the recessive allele; determine each parent's genotype and phenotype; draw a tic-tac-toe grid; place one parent's genotype across the top and the other parent's genotype down the left side, one letter per square; combine the letters from the top and left to fill the four inner squares.

Punnett Square: Heterozygous Tall Parents (Tt × Tt)

Tall (T) is dominant and short (t) is recessive; offspring have a 75% chance of being tall and a 25% chance of being short; genotypes are 25% TT, 50% Tt, and 25% tt.

Crossing Over and Recombinant Chromosomes

Genes on the same chromosome are linked and can be passed to daughter cells as one unit; during crossing over, homologous chromosomes can break even between linked genes and precisely exchange gene segments; this produces recombinant chromosomes containing a mixture of contributions from each parent and increases genetic variability.

Random Fertilization

A single egg is fertilized by a single sperm in a random manner.

Genetic Possibilities from Fertilization

Independent assortment and random fertilization together produce approximately 72 trillion possible zygote combinations (8.5 million egg possibilities × 8.5 million sperm possibilities); crossing over increases the number of possibilities exponentially; the odds of any one person being exactly who they are are greater than 1 in 72,000,000,000.

Complexity of Heredity

Few phenotypes can be traced to a single gene; most traits are determined by multiple alleles or interactions among several pairs of genes.

Dominant Inheritance

Traits carried on dominant alleles; a dominant allele usually masks or suppresses a recessive allele; examples include widow's peaks, freckles, dimples, double-jointed thumbs, ability to roll the tongue, and ability to taste phenylthiocarbamide (PTC).

Dominant Inheritance: Heterozygous (Aa)

The dominant trait is expressed; there is a 50% chance the dominant allele will be passed to offspring.

Dominant Inheritance: Homozygous Dominant (AA)

The dominant trait is expressed; there is a 100% chance the dominant allele will be passed to offspring.

Dominant Inheritance: Homozygous Recessive (aa)

The dominant trait is not present and cannot be passed on because the individual has no dominant allele.

Dominant Genetic Disorders

Uncommon because many are lethal and cause death before reproductive age; affected individuals cannot be unaffected carriers because the dominant allele is expressed, and people may consciously decide not to have children to avoid passing on the disorder; examples include syndactyly (webbed digits), achondroplasia (heterozygous individuals have dwarfism while homozygous dominant is lethal), and Huntington's disease (a delayed-action gene that is not activated until approximately age 40).

Recessive Inheritance

Traits carried on recessive alleles; examples listed in the slides include normal digits, normal endochondral ossification, and absence of Huntington's disease.

Recessive Inheritance: Heterozygous (Aa)

The recessive trait is not expressed; the individual is a carrier and has a 50% chance of passing the recessive allele to offspring.

Recessive Inheritance: Homozygous Dominant (AA)

The recessive trait is not present and cannot be passed on because the individual has no recessive allele.

Recessive Inheritance: Homozygous Recessive (aa)

The recessive trait is expressed; there is a 100% chance the recessive allele will be passed on to offspring.

Recessive Genetic Disorders

More common than dominant disorders because heterozygous individuals can carry recessive alleles without expressing the disorder; unless genetic testing is done or there is a family history, most carriers do not know they are carriers and may unknowingly pass the disorder to their children; examples include albinism, Tay-Sachs disease, and cystic fibrosis.

Autosomal Recessive Carrier Parents (Aa × Aa)

Each pregnancy has a 25% chance of producing an affected child (aa).

Incomplete Dominance

Occurs when heterozygous individuals have an intermediate phenotype; they may have symptoms, but usually not as intensely as homozygous individuals.

Sickle-Cell Inheritance

The slides describe the sickling gene as an example of incomplete dominance: homozygous dominant (SS) produces normal hemoglobin and does not have sickle-cell trait; heterozygous (Ss) produces both normal and mutated hemoglobin and has sickle-cell trait; homozygous recessive (ss) produces only mutated hemoglobin and has sickle-cell anemia.

Sickle-Cell Crisis

Individuals with sickle-cell trait (Ss) can experience a sickle-cell crisis during prolonged reduction in blood oxygen; individuals with sickle-cell anemia (ss) are more susceptible to a crisis even with a short reduction in oxygen.

Multiple Alleles

A single trait controlled by three or more different alleles; human ABO blood type is an example, with three alleles: IA, IB, and i.

Codominance

Occurs when both phenotypes are expressed in a heterozygous individual; in human ABO blood groups, IA and IB are both dominant and are codominant with each other, while i is recessive.

ABO Blood Type Genotypes

Type O = ii (homozygous recessive); Type A = IAIA (homozygous dominant) or IAi (heterozygous); Type B = IBIB (homozygous dominant) or IBi (heterozygous); Type AB = IAIB, in which both A and B are expressed through codominance.

Rh Blood Group Inheritance

Rh factor is inherited independently of ABO blood type; Rh-positive (Rh+) is dominant and Rh-negative (Rh−) is recessive; this is an example of complete dominance, in which the dominant allele masks the recessive allele.

Sex Chromosomes and Genetic Sex

Chromosome pair 23 contains the sex chromosomes, which determine genetic sex and code for other traits; females are XX and can only pass an X chromosome to offspring; males are XY and can pass either an X or Y chromosome, so the father's contribution determines the offspring's genetic sex.

Sex-Linked Inheritance

Inherited traits determined by genes located on the sex chromosomes; the X chromosome carries over 1,400 genes, many coding for proteins important for brain function, while the Y chromosome carries about 200 genes; genes found only on the X chromosome are called X-linked genes.

X-Linked Recessive Inheritance

X-linked recessive conditions can be passed from mothers to sons; examples include hemophilia and red-green color blindness; females need two recessive alleles, one on each X chromosome, to express the condition; a carrier mother with an unaffected father has a 50% chance of having an affected son.

Why X-Linked Recessive Traits Are Expressed in Males

Males have only one X chromosome and no corresponding allele on the Y chromosome to mask an X-linked recessive allele, so the allele is always expressed; females need recessive alleles on both X chromosomes to express an X-linked condition.