Biology: Chromosomes and Inheritance - PDF

**Chromosomes and Inheritance** Introduction In [eukaryotic](javascript:void(0)) organisms (eg, humans), genes are found on chromosomes within the nucleus and on DNA present within mitochondria. Therefore, to understand inheritance patterns of genetic traits, chromosome (and mitochondrion) transm...

**Chromosomes and Inheritance** Introduction In [eukaryotic](javascript:void(0)) organisms (eg, humans), genes are found on chromosomes within the nucleus and on DNA present within mitochondria. Therefore, to understand inheritance patterns of genetic traits, chromosome (and mitochondrion) transmission from parents to offspring, as well as factors that affect chromosome structure and function, must be considered. This lesson deals with sex determination in humans, the inheritance of sex-linked traits, and the concepts of genetic linkage and gene mapping. In addition, this lesson describes the inheritance of extra-nuclear genes (ie, genes present outside the nucleus) and covers the various kinds of mutations that can affect genetic inheritance. 7.3.01 Determination of Sex In mammals, **genotypic sex** is determined by an inherited combination of [sex chromosomes](javascript:void(0)). **Females (XX)** typically inherit one maternal X chromosome and one paternal X chromosome. **Males (XY)** typically inherit one maternal X chromosome and one paternal Y chromosome. Structurally, the Y chromosome is considerably shorter than the X chromosome and contains fewer genes, some of which are not present on the X chromosome. One of the genes typically present on the Y chromosome but not on the X chromosome is *SRY*, which plays an essential role in mammalian sex determination. Expression of *SRY* induces the development of the male gonads (testes). When the SRY protein is produced, fetuses generally develop a male phenotype, but in the absence of SRY, fetuses generally develop a female phenotype (see Figure 7.18). Chapter 7: Genetics 288 **Figure 7.18** Typical mechanism of sex determination in humans. 7.3.02 Sex-Linked Traits In **sex-linked inheritance**, traits arise from genes on [sex chromosomes](javascript:void(0)) (ie, X and Y chromosomes). A person who inherits two X chromosomes typically has female characteristics, whereas a person who inherits one X and one Y chromosome typically has male characteristics. Therefore, XX and XY individuals exhibit different inheritance patterns for sex-linked traits. [Recessive X-linked traits](javascript:void(0)) are exhibited in XY individuals more often than in XX individuals because in XY individuals, expression of recessive alleles on the X chromosome cannot typically be [masked](javascript:void(0)) by alleles on the Y chromosome. In contrast, as shown in Figure 7.19, an XX individual who inherits only one copy of a recessive X-linked allele is a **carrier** of the trait (ie, unaffected heterozygote). For an XX individual to *exhibit* a recessive X-linked trait, the person must inherit the recessive alleles on *both* X chromosomes. A diagram of a female reproductive system Description automatically generated Chapter 7: Genetics 289 **Figure 7.19** Inheritance of an X-linked recessive disorder. In addition, because the number of genes unique to the Y chromosome is small, [Y-linked traits](javascript:void(0)) are rare, and those that do occur appear exclusively in individuals with a Y chromosome. 7.3.03 Genetic Linkage For [alleles](javascript:void(0)) on different chromosomes, the law of independent assortment states that when the alleles separate during [meiosis](javascript:void(0)), they will do so [independently](javascript:void(0)) of each other (see Concept 7.1.03). However, when genes are close together on the same chromosome, the genes are **linked** and do *not* assort independently into gametes. **Genetic linkage** refers to the tendency of alleles on the same chromosome to remain on the same chromosome, as shown in Figure 7.20. Linked alleles do not separate independently during meiosis unless a [crossing over event](javascript:void(0)) happens between them. As genes become closer in proximity on the same chromosome, the frequency of crossing over between them decreases, and the genes are described as being tightly or closely linked.  Chapter 7: Genetics 290 **Figure 7.20** Genetic linkage. 7.3.04 Gene Mapping [Homologous chromosomes](javascript:void(0)) can exchange genetic information in a process called [crossing over](javascript:void(0)), or recombination, during prophase I of meiosis (see Concept 7.1.02). Crossing over can produce new combinations of alleles within a chromosome by moving some alleles of the maternal copy to the paternal copy and vice versa. New combinations of alleles are called **recombinant**, whereas combinations that already existed in a parent are called **parental**. If two genes are located close together on a chromosome, they are relatively unlikely to be separated by a recombination event because there is little distance between the two genes in which crossing over can occur. Consequently, **recombination frequency** provides an indication of the physical distance between two genes on a chromosome (see Figure 7.21). Analysis of recombination frequencies can be used to construct a **gene map**, which depicts relative positions of genes on a chromosome. Distances between genes are reported in [map units](javascript:void(0)), or centimorgans, with a 1% recombination frequency equal to 1 map unit. A diagram of different types of pronoun AI-generated content may be incorrect. Chapter 7: Genetics 291 **Figure 7.21** Gene map based on recombination frequencies. 7.3.05 Biometry Analysis of biological data involves the application of statistical and mathematical methods. The use of such methods in biological sciences is referred to as **biometry**, or biostatistics. In addition to its role in data analysis, biometry is essential to researchers during the process of experimental design. Biometry has been particularly important in the development of the field of genetics, beginning with Gregor Mendel\'s quantitative approach to analyzing the inheritance of traits in pea plants during the mid-1800s (see Lesson 7.2). The chi-square test is an example of a statistical test used in biological sciences. The chi-square test can be used to evaluate a [null hypothesis](javascript:void(0)) (H0), for example, the hypothesis that two genes assort independently, as shown in Figure 7.22.  Chapter 7: Genetics 292 **Figure 7.22** Example of the application of biometry. 7.3.06 Extranuclear Inheritance Patterns **Mitochondria** are [cellular organelles](javascript:void(0)) that have their own DNA genome, which contains a very small number of genes compared to the number of genes located on chromosomes within the nucleus. Because [mitochondrial genes](javascript:void(0)) are **extranuclear** (ie, located outside the nucleus), they are inherited in a distinctly different manner than nuclear genes (ie, genes on autosomes and sex chromosomes). During fertilization, mitochondria within sperm pass into the ovum but typically are eliminated during early embryonic development (Figure 7.23). Therefore, only *maternal* mitochondria typically persist in offspring, resulting in an inheritance pattern in which [transmission](javascript:void(0)) of mitochondrial traits is via the mother only. A diagram of flowers with white text AI-generated content may be incorrect. Chapter 7: Genetics 293 **Figure 7.23** Transmission of mitochondrial DNA. 7.3.07 DNA Mutations [Mutations](javascript:void(0)) are changes in DNA sequences that result from errors in [DNA replication](javascript:void(0)), faulty [DNA repair](javascript:void(0)), or exposure to physical or chemical factors (ie, mutagens) that interact with DNA and affect its structure. Mutations can be broadly characterized as **point mutations**, in which a single nucleotide pair is altered, or **chromosomal mutations**, which involve larger portions of the DNA molecule. In addition, [frameshift](javascript:void(0)) [mutations](javascript:void(0)) occur as a result of the insertion or deletion of a nucleotide (or of a small number of nucleotides that is not a multiple of three) that changes the [reading frame](javascript:void(0)) of the DNA molecule. As shown in Figure 7.24, point mutations include: [Missense mutations](javascript:void(0)), in which a nucleotide substitution in the DNA sequence results in the substitution of one amino acid for another in the polypeptide encoded by the DNA. [Nonsense mutations](javascript:void(0)), in which the nucleotide change results in the formation of a stop codon from a codon that previously encoded an amino acid. [Silent mutations](javascript:void(0)), in which a nucleotide substitution does not alter the amino acid sequence of the encoded polypeptide in any way.  Chapter 7: Genetics 294 **Figure 7.24** Different types of point mutations. Different types of point mutations, as well as frameshift mutations and possible effects of each mutation type, are discussed in further detail in Lesson 2.3. As previously mentioned, chromosomal mutations are more extensive than point mutations. Chromosomal mutations include (Figure 7.25): **Deletion** mutations, in which a section of DNA is removed from a chromosome. **Duplication** mutations, in which a section of DNA is repeated on a chromosome. **Inversion** mutations, in which a section of DNA breaks from a chromosome and reattaches in the reverse orientation. **Translocation** mutations, in which a section of DNA breaks from one chromosome and attaches to another chromosome. A diagram of different types of dna AI-generated content may be incorrect. Chapter 7: Genetics 295 **Figure 7.25** Different types of chromosomal mutations. Unlike point mutations, which typically affect a single gene, chromosomal mutations can potentially disrupt the function of numerous genes (depending on the size of the chromosomal region affected). Therefore, chromosomal mutations are usually harmful. Some chromosomal mutations, such as inversions and translocations, do not immediately cause a gain or loss of genetic information but can lead to altered expression of the genes involved. Such mutations can cause phenotypic effects because a gene\'s position on a chromosome relative to other genes and genetic control elements (eg, promotors, enhancers) can affect its expression. 7.3.08 Mutagens Mutations are changes to the nucleotide sequence that makes up an organism\'s [genome](javascript:void(0)). Mutations can occur spontaneously or be caused by **mutagens**, agents that promote genetic changes or increase their frequency. As shown in Table 7.2, mutagens can be generally categorized as physical, chemical, or biological agents.  Chapter 7: Genetics 296 **Table 7.2** Types of mutagens. **Type of mutagen** **Examples** Physical Ionizing radiation Non-ionizing radiation Heat Chemical Reactive chemicals Base analogs Intercalating agents Biological Viruses Bacteria Transposable elements **Physical mutagens** include heat and various forms of radiation (eg, X-rays, gamma rays, ultraviolet light). Exposure to physical mutagens can result in DNA damage including strand breaks and [pyrimidine](javascript:void(0)) [dimers](javascript:void(0)) (ie, distortion of the DNA molecule via formation of covalent bonds between adjacent [pyrimidine](javascript:void(0)) [bases](javascript:void(0))). **Chemical mutagens** can cause mutations in a variety of ways. Some chemical mutagens react directly with DNA and cause structural changes. Other chemicals (ie, base analogs) are structurally similar to [DNA bases](javascript:void(0)) and can be incorporated into DNA during replication, which can result in base mispairing. In addition, some chemical mutagens (ie, intercalating agents) become inserted between DNA bases, which can cause frameshift mutations during DNA replication. **Biological mutagens** include certain viruses (eg, human papilloma virus) and bacteria (eg, *Helicobacter pylori*). Viruses can cause mutations via [insertion](javascript:void(0)) of the viral genome into the host cell genome, while bacterial infections may result in chronic inflammatory conditions that favor mutations. The movement of mobile genetic elements (eg, [transposons](javascript:void(0)), [retrotransposons](javascript:void(0))) within the genome can also generate mutations. 7.3.09 Somatic and Germline Mutations The cells that make up an animal can be classified as either **germ cells** (reproductive cells) or **somatic cells** (all other cells). Germ cells are progenitor cells that undergo [meiosis](javascript:void(0)) to produce [gametes](javascript:void(0)) (ie, sperm, ova \[eggs\]). Mutations (ie, heritable changes in DNA sequences) can occur in both germ cells and somatic cells. Mutations that occur in parental germ cells are called **germline mutations** (Figure 7.26). Such mutations can be passed to offspring via sexual reproduction because gametes, which are derived from germ cells, combine to form the [zygote](javascript:void(0)) (ie, initial cell of the offspring). In contrast, parental **somatic mutations**, which involve alteration of somatic cell DNA, do *not* pass to offspring because somatic cells are not directly involved in zygote formation (ie, somatic cells do not pass from parents to offspring). Chapter 7: Genetics 297 **Figure 7.26** Germline mutations versus somatic mutations. 7.3.10 Outcomes of Mutations **Mutations** are changes in the nucleotide sequence of DNA molecules. Such changes can produce a wide range of effects in organisms. [Natural selection](javascript:void(0)) is an [evolutionary mechanism](javascript:void(0)) by which [adaptive traits](javascript:void(0)) that increase the [fitness](javascript:void(0)) (reproductive success) of an organism are more likely than less favorable traits to be passed to the next generation. This tendency causes **beneficial mutations** (those found in adaptive alleles) to become more common in a population over time and **detrimental mutations** to become less common (Table 7.3). Diagram of germination and fetus AI-generated content may be incorrect. Chapter 7: Genetics 298 **Table 7.3** Effects of different types of mutations. Some types of mutations are more likely than others to cause harmful effects in organisms. In general, frameshift mutations and nonsense mutations are more likely than missense mutations to cause harmful effects because frameshift and nonsense mutations are more likely than missense mutations to cause the production of nonfunctional protein molecules. Conversely, silent mutations, which do not alter the amino acid sequence of encoded proteins, do not affect an organism\'s fitness (ie, are evolutionarily neutral). **Inborn errors of metabolism** are genetic disorders caused by detrimental mutations in genes that code for metabolic enzymes. Because enzyme activity is decreased, these disorders result in the accumulation of metabolites upstream of the affected enzyme and decreased levels of the downstream products in the pathway. The resulting changes in metabolite levels frequently have negative clinical effects such as lethargy, impaired development, and toxicity.  **Have you mastered this Lesson?** Mark this lesson as complete and continue to the next.